GTK: add a comment better suiting GCC 7 parsing

Change-Id: Iab1eb1b86a750f3dedce6b2bd467fb3ba98243df
Reviewed-on: https://code.wireshark.org/review/22613
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>

ui/gtk/gui_utils.c

@@ -1281,7 +1281,7 @@ tree_view_key_pressed_cb(GtkWidget   *tree,
                 rc = TRUE;
                 break;
             }
-            /* else fall through to jumping to the parent */
+            /* else fall through */
         case GDK_BackSpace:
             if (!expanded) {
                 /* subtree is already collapsed, jump to parent node */