tools/include: fix (drop) dependencies of when to populate xen/

tools/include: fix (drop) dependencies of when to populate xen/

Making the population of xen/ depend on the time stamps of a subset of
the headers put there is error prone. The creation of a few dozen
symlinks doesn't take a meaningful amount of time (compared to the
overall building of tools/), and hence - to be on the safe side - should
simply be done always. Convert the goal to a phony one and drop its
dependencies, effectively taking further what 8d8d7d6b3dc1 ("tools: fix
linking hypervisor includes to tools include directory") had already
attempted.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Wei Liu <wl@xen.org>