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

contrib/binutils/include/libiberty.h

@@ -102,7 +102,7 @@ extern int writeargv PARAMS ((char **, FILE *));
    is 1, we found it so don't provide any declaration at all.  */
 #if !HAVE_DECL_BASENAME
 #if defined (__GNU_LIBRARY__ ) || defined (__linux__) || defined (__FreeBSD__) || defined (__OpenBSD__) || defined(__NetBSD__) || defined (__CYGWIN__) || defined (__CYGWIN32__) || defined (__MINGW32__) || defined (HAVE_DECL_BASENAME)
-extern char *basename (const char *);
+#include <libgen.h>
 #else
 /* Do not allow basename to be used if there is no prototype seen.  We
    either need to use the above prototype or have one from