Nothing in defs.h actually uses this.
Add some includes for some spots using things from hashtab.h. Note that
if the GDB build doesn't use libxxhash, hashtab.h is included by
gdbsupport/common-utils.h, so all files still see hashtab.h. It puzzled
me for some time why I didn't see build failures in my build (which
didn't use libxxhash) but the buildbot gave build failures (it uses
libxxhash).
Change-Id: I8efd68decdaf579f048941c7537cd689885caa2a
Approved-By: John Baldwin <jhb@FreeBSD.org>
740ccea39e