X11: Fix XKB events being passed on to core path

(cherry picked from commit a17a1b2011)

src/x11_window.c

@@ -1195,6 +1195,8 @@ static void processEvent(XEvent *event)
             {
                 _glfw.x11.xkb.group = ((XkbEvent*) event)->state.group;
             }
+
+            return;
         }
     }