Fix missing regex flags

Related feedback:
- https://www.reddit.com/r/uBlockOrigin/comments/1005a8f/suddenly_my_filters_doesnt_work/j2htwof/

src/js/contentscript-extra.js

@@ -36,7 +36,7 @@ const nonVisualElements = {
 
 const regexFromString = (s, exact = false) => {
     if ( s === '' ) { return /^/; }
-    const match = /^\/(.+)\/([i]?)$/.exec(s);
+    const match = /^\/(.+)\/([imu]*)$/.exec(s);
     if ( match !== null ) {
         return new RegExp(match[1], match[2] || undefined);
     }