kbuild: clean Modules.symvers in external module dirs

At the moment, running `make clean` in an external module directory does a
nice job of cleaning up with one exception: it leaves behind Modules.symvers.
Attached patch adds this file to the clean list for external modules.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1255,8 +1255,10 @@ $(clean-dirs):
 	$(Q)$(MAKE) $(clean)=$(patsubst _clean_%,%,$@)
 
 clean:	rm-dirs := $(MODVERDIR)
+clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers
 clean: $(clean-dirs)
 	$(call cmd,rmdirs)
+	$(call cmd,rmfiles)
 	@find $(KBUILD_EXTMOD) $(RCS_FIND_IGNORE) \
 		\( -name '*.[oas]' -o -name '*.ko' -o -name '.*.cmd' \
 		-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \) \