Fix LESS strict math in lists

src/assets/font-awesome/less/list.less

@@ -17,6 +17,6 @@
   top: (2em / 14);
   text-align: center;
   &.@{fa-css-prefix}-lg {
-    left: -@fa-li-width + (4em / 14);
+    left: (-@fa-li-width + (4em / 14));
   }
 }