compositor: Pass weston_output as the frame_signal argument

The frame time is in weston_output, so passing weston_output itself is
more useful.
diff --git a/src/compositor-wayland.c b/src/compositor-wayland.c
index ecaee06..cde7658 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -347,7 +347,7 @@
 
 	draw_border(output);
 
-	wl_signal_emit(&output->frame_signal, &output->frame_time);
+	wl_signal_emit(&output->base.frame_signal, output);
 
 	eglSwapBuffers(compositor->base.egl_display, output->egl_surface);
 	callback = wl_surface_frame(output->parent.surface);