Add inline display to list item avatar icons.

This makes order of importing the material icons
stylesheet irrelevant. Forcing icons to display
inline to avoid mis-alignment.

src/list/_list.scss

@@ -44,6 +44,7 @@
       line-height: 0;
       vertical-align: middle;
       padding: 0 12px 0 0;
+      display: inline;
     }
   }