[bugfix][makefile] fix plugin dir location
[bugfix][makefile] fix plugin dir location

https://github.com/open-falcon/falcon-plus/pull/404

open-falcon/falcon-plus,open-falcon/falcon-plus,open-falcon/falcon-plus,taomaree/falcon-plus,taomaree/falcon-plus,taomaree/falcon-plus,open-falcon/falcon-plus,taomaree/falcon-plus,open-falcon/falcon-plus,taomaree/falcon-plus,taomaree/falcon-plus