Replace local prototype of basename() with an inclusion of <libgen.h>.

libiberty currently defines the prototype for basename() itself instead
of using <libgen.h>. It still uses the BSD-style prototype instead of
the POSIX one, meaning that if FreeBSD would switch over to the POSIX
one, you wouldn't be able to use libiberty.h and libgen.h in a single
source file. It turns out that kgdb does this. Patch up libiberty to
just include <libgen.h>.

I'm currently talking to upstream to see whether we can come up with a
more complete solution that could be integrated, but for our
unmaintained copy of GDB in base, let's just apply the simplest
workaround possible.

Reviewed by: pfg
Differential Revision: https://reviews.freebsd.org/D6631