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.