[linter] Allow tabs in lib/codemirror.css

Since rollup is now adding those

test/lint.js

@@ -4,7 +4,8 @@ var blint = require("blint");
   blint.checkDir(dir, {
     browser: true,
     allowedGlobals: ["CodeMirror", "define", "test", "requirejs"],
-    ecmaVersion: 5
+    ecmaVersion: 5,
+    tabs: dir == "lib"
   });
 });