Provide an alias to _obstack_free

Provide an alias to _obstack_free

GDB 7.11 does not build if compiled against uclibc-ng (1.0.12 was used,
but the problematic code exists in 1.0.13 as well). The reason is that
GDB release includes its own obstack implementation, but elides the code
if <gnu-versions.h> declares a compatible obstack implementation in
libc. uclibc-ng does claim compatible obstack interface (GDB expects
version 2, but accepts version 1 if sizeof(int) == sizeof(size_t)),
however, uclibc-ng does not provide the _obstack_free symbol that is a
part of the interface (glibc does provide this symbol). This later
causes a link failure.

The attached patch makes uclibc-ng enables an alias _obstack_free ->
obstack_free.

Signed-off-by: Alexey Neyman <stilor@att.net>