egl/wayland: fix return value in dri2_wl_swrast_commit_backbuffer

The function returns "void" rather than int. We could rework that, yet
again there will be no benefit since all the callers have no use of it.

Fixes: 9ca6711faa ("Revert "wayland: Block for the frame callback in
get_back_bo not dri2_swap_buffers"")
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/egl/drivers/dri2/platform_wayland.c

@@ -1559,7 +1559,7 @@ dri2_wl_swrast_commit_backbuffer(struct dri2_egl_surface *dri2_surf)
    while (dri2_surf->throttle_callback != NULL)
       if (wl_display_dispatch_queue(dri2_dpy->wl_dpy,
                                     dri2_dpy->wl_queue) == -1)
-         return -1;
+         return;
 
    if (dri2_surf->base.SwapInterval > 0) {
       dri2_surf->throttle_callback =