xen/include: compat/xlat.h may change with .config changes

xen/include: compat/xlat.h may change with .config changes

$(xlat-y) getting derived from $(headers-y) means its contents may
change with changes to .config. The individual files $(xlat-y) refers
to, otoh, may not change, and hence not trigger rebuilding of xlat.h.
(Note that the issue was already present before the commit referred to
below, but it was far more limited in affecting only changes to
CONFIG_XSM_FLASK.)

Fixes: 2c8fabb2232d ("x86: only generate compat headers actually needed")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>