uml: x86: add FORCE to user_constants.h

The build system has started warning when filechk is called
without FORCE:

arch/x86/um/Makefile:44: FORCE prerequisite is missing

Add FORCE to make sure the file is checked/rebuilt when
necessary (and to quiet up the warning.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>