Makefile: add target to print generated files

This is helpful for automatic code analysis.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

Makefile

@@ -1045,6 +1045,9 @@ endif
 include $(SRC_PATH)/tests/docker/Makefile.include
 include $(SRC_PATH)/tests/vm/Makefile.include
 
+printgen:
+	@echo $(GENERATED_FILES)
+
 .PHONY: help
 help:
 	@echo  'Generic targets:'