Fix broken compilation of uClibc-ng.

Fix broken compilation of uClibc-ng.

During buildroot compilation with latest uClibc
I've encoutered linking error due to multiple definition
of some symbols from DNS code.
The error happens because the same file resolv.c
is included inside many other .c files:
res_comp.c:(.text+0x0): multiple definition of `__GI___dn_expand'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x8a0): first defined here
res_comp.c:(.text+0x0): multiple definition of `__dn_expand'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x8a0): first defined here
res_comp.c:(.text+0x34): multiple definition of `__GI___dn_comp'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0xc68): first defined here
res_comp.c:(.text+0x34): multiple definition of `__dn_comp'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0xc68): first defined here
ns_name.c:(.text+0x4c): multiple definition of `__GI___ns_name_ntop'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x4c): first defined here
ns_name.c:(.text+0x4c): multiple definition of `__ns_name_ntop'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x4c): first defined here
ns_name.c:(.text+0x1f8): multiple definition of `__GI___ns_name_pton'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x1f8): first defined here
ns_name.c:(.text+0x1f8): multiple definition of `__ns_name_pton'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x1f8): first defined here
ns_name.c:(.text+0x624): multiple definition of `__hnbad'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x624): first defined here
ns_name.c:(.text+0x718): multiple definition of `__GI___ns_name_unpack'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x718): first defined here
ns_name.c:(.text+0x718): multiple definition of `__ns_name_unpack'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x718): first defined here
ns_name.c:(.text+0x84c): multiple definition of `__GI___ns_name_uncompress'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x84c): first defined here
ns_name.c:(.text+0x84c): multiple definition of `__ns_name_uncompress'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x84c): first defined here
ns_name.c:(.text+0x8a0): multiple definition of `__GI___ns_name_pack'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x8d4): first defined here
ns_name.c:(.text+0x8a0): multiple definition of `__ns_name_pack'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0x8d4): first defined here
ns_name.c:(.text+0xbe4): multiple definition of `__GI___ns_name_compress'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0xc18): first defined here
ns_name.c:(.text+0xbe4): multiple definition of `__ns_name_compress'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0xc18): first defined here
ns_name.c:(.text+0xc34): multiple definition of `__GI___ns_name_skip'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0xcdc): first defined here
ns_name.c:(.text+0xc34): multiple definition of `__ns_name_skip'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0xcdc): first defined here
ns_name.c:(.text+0xcd4): multiple definition of `__GI___dn_skipname'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0xd7c): first defined here
ns_name.c:(.text+0xcd4): multiple definition of `__dn_skipname'; libc/libc_so.a(encodeq.os):encodeq.c:(.text+0xd7c): first defined here
My previous commit that fixes build error of DNS code is okay,
but there are some 'bottlenecks' in uClibc-ng code, so if
we don't want to completely rewrite resolv.c we need to make some
symbols weak to prevent linking errors.