build system: create .xcompile dependency

It's probably safe to say that .xcompile needs an update if
util/xcompile/xcompile changed, so tell make about this
dependency.

Updates are honored immediately due to GNU make's feature of
reinterpreting everything when an included file changes.  See "How
Makefiles Are Remade" in the GNU make documentation for details.

Change-Id: Ide2f028eaddcee66028c6403688cc83e1622fa6b
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6255
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>