rules/mak: make clean should blow away timestamp files

Using a global pattern makes it easier to clean out
old generated files.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

rules.mak

@@ -88,6 +88,11 @@ config-%.h: config-%.h-timestamp
 config-%.h-timestamp: config-%.mak
 	$(call quiet-command, sh $(SRC_PATH)/scripts/create_config < $< > $@, "  GEN   $(TARGET_DIR)config-$*.h")
 
+.PHONY: clean-timestamp
+clean-timestamp:
+	rm -f *.timestamp
+clean: clean-timestamp
+
 # will delete the target of a rule if commands exit with a nonzero exit status
 .DELETE_ON_ERROR: