Mark internal nscd functions with attribute_hidden [BZ #18822]

Mark internal nscd functions with attribute_hidden [BZ #18822]

Mark internal nscd functions with attribute_hidden to allow direct
access within libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* nscd/nscd-client.h (__nscd_get_map_ref): Add attribute_hidden.
	(__nscd_unmap): Likewise.
	(__nscd_cache_search): Likewise.
	(__nscd_get_nl_timestamp): Likewise.
	(__nscd_getpwnam_r): Likewise.
	(__nscd_getpwuid_r): Likewise.
	(__nscd_getgrnam_r): Likewise.
	(__nscd_getgrgid_r): Likewise.
	(__nscd_gethostbyname_r): Likewise.
	(__nscd_gethostbyname2_r): Likewise.
	(__nscd_gethostbyaddr_r): Likewise.
	(__nscd_getai): Likewise.
	(__nscd_getgrouplist): Likewise.
	(__nscd_getservbyname_r): Likewise.
	(__nscd_getservbyport_r): Likewise.
	(__nscd_innetgr): Likewise.
	(__nscd_setnetgrent): Likewise.