gdb: don't include hashtab.h in defs.h

gdb: don't include hashtab.h in defs.h

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>