kbuild: Add build dep for UML

Using .S -> .s needs scripts/fixdep, which is not necessarily there
so early during the build, so add a dependency on it.

arch/um/Makefile

@@ -71,7 +71,7 @@ LDFLAGS_vmlinux = -r $(ARCH_DIR)/main.o
 vmlinux: $(ARCH_DIR)/main.o 
 
 
-$(ARCH_DIR)/uml.lds.s : $(ARCH_DIR)/uml.lds.S
+$(ARCH_DIR)/uml.lds.s : $(ARCH_DIR)/uml.lds.S scripts FORCE
 	$(call if_changed_dep,as_s_S)
 
 AFLAGS_uml.lds.o = -U$(SUBARCH) -DSTART=$$(($(TOP_ADDR) - $(SIZE))) \