kbuild: Fix parallel build in a distclean'ed tree

Fixes the following error:
make: *** No rule to make target `.tmp_kallsyms2.S', needed by `.tmp_kallsyms2.o'.

Problem is that make does not know it have to visit scripts before it can use $(KALLSYMS)
$(KALLSYMS) is a dependency to .tmp_kallsyms% but make suddenly complains about
.tmp_kallsyms2 for some reasons.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -600,6 +600,9 @@ cmd_kallsyms = $(NM) -n $< | $(KALLSYMS) $(foreach x,$(CONFIG_KALLSYMS_ALL),--al
 .tmp_vmlinux3: $(vmlinux-objs) .tmp_kallsyms2.o arch/$(ARCH)/kernel/vmlinux.lds FORCE
 	$(call if_changed_rule,vmlinux__)
 
+# Needs to visit scripts/ before $(KALLSYMS) can be used.
+$(KALLSYMS): scripts ;
+
 endif
 
 # Finally the vmlinux rule