Don't declare xmalloc or xrealloc in bucomm.h

Don't declare xmalloc or xrealloc in bucomm.h

It's better to include the proper header, which has declarations with
various attributes.  Commit 096aefc040 in 1994 introduced this wart.

	* bucomm.h (xmalloc, xrealloc): Delete declaration.
	* od-macho.c: Include libiberty.h.
	* od-xcoff.c: Include libiberty.h.