compositor: Send ping to keyboard focus surface for key press

We were sending to the pointer focus surface.
diff --git a/src/compositor.c b/src/compositor.c
index eb795a2..a4ef26b 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -1849,7 +1849,7 @@
 	struct weston_seat *ws = (struct weston_seat *) seat;
 	struct weston_compositor *compositor = ws->compositor;
 	struct weston_surface *focus =
-		(struct weston_surface *) seat->pointer->focus;
+		(struct weston_surface *) seat->keyboard->focus;
 	struct wl_keyboard_grab *grab = seat->keyboard->grab;
 	uint32_t serial = wl_display_next_serial(compositor->wl_display);
 	uint32_t *k, *end;