um: Generate headers before generating user-offsets.s

In case we need generated header files for the values in
user-offsets.h, make sure we build generated header files before
user-offsets.s is built.
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>

arch/um/Makefile

@@ -140,7 +140,7 @@ archclean:
 
 # Generated files
 
-$(HOST_DIR)/um/user-offsets.s: FORCE
+$(HOST_DIR)/um/user-offsets.s: __headers FORCE
 	$(Q)$(MAKE) $(build)=$(HOST_DIR)/um $@
 
 define filechk_gen-asm-offsets