src/Makefile: remove css directory in make clean

Author: adtac

src/Makefile

@@ -2,7 +2,7 @@ BROWSERIFY := node_modules/.bin/browserify
 PRETTIER := node_modules/.bin/prettier
 LESSC := node_modules/.bin/lessc
 
-CLEAN_FILES := js
+CLEAN_FILES := js css
 PRETTIER_FILES := $(wildcard *.json *.js popup/*.js options/*.js *.less popup/*.less options/*.less *.html popup/*.html options/*.html)
 
 .PHONY: all