Fix "make mrproper" that broke when the files pattern matched

a directory pattern. Clean directories _first_, then files.

Makefile

@@ -732,8 +732,8 @@ mrproper: clean archmrproper
 	@find . $(RCS_FIND_IGNORE) \
 		\( -name .depend -o -name .\*.cmd \) \
 		-type f -print | xargs rm -f
-	@rm -f $(MRPROPER_FILES)
 	@rm -rf $(MRPROPER_DIRS)
+	@rm -f $(MRPROPER_FILES)
 	@$(MAKE) -C scripts mrproper
 	@$(MAKE) -f Documentation/DocBook/Makefile mrproper