add styles to .select2-drop-mask needed for mask to work in IE

Author: andrewalcantara

select2.css

@@ -144,6 +144,10 @@ Version: @@ver@@ Timestamp: @@timestamp@@
     left: 0;
     top: 0;
     z-index: 9998;
+    /* styles required for IE to work */
+    background-color: #fff;
+    opacity: 0;
+    filter: alpha(opacity=0);
 }
 
 .select2-drop {