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>

arch/x86/um/Makefile

@@ -40,7 +40,7 @@ $(obj)/user-offsets.s: c_flags = -Wp,-MD,$(depfile) $(USER_CFLAGS) \
 	-Iarch/x86/include/generated
 targets += user-offsets.s
 
-include/generated/user_constants.h: $(obj)/user-offsets.s
+include/generated/user_constants.h: $(obj)/user-offsets.s FORCE
 	$(call filechk,offsets,__USER_CONSTANT_H__)
 
 UNPROFILE_OBJS := stub_segv.o