Makefile: Add back TAGS/ctags/cscope rules

It is a bit of a pain to be forced to run configure before being able
to use cscope and friends. Add back the rules to build them in-tree
as before commit a5665051.

Fixes: a5665051 ("configure: integrate Meson in the build system")
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <159897001005.442705.16516671603870288336.stgit@bahia.lan>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>