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: H. Peter Anvin <hpa@linux.intel.com>
diff --git a/arch/um/Makefile b/arch/um/Makefile
index 00210e9..9548ddc 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -140,7 +140,7 @@
 
 # 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