MAINTAINERS: add reviewers for virtio-gpu

Add Gurchetan Singh and me as reviewers for virtio-gpu.
Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Acked-by: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20211028213446.955338-1-olvaffe@gmail.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

MAINTAINERS

@@ -19906,6 +19906,8 @@ F:	include/uapi/linux/virtio_gpio.h
 VIRTIO GPU DRIVER
 M:	David Airlie <airlied@linux.ie>
 M:	Gerd Hoffmann <kraxel@redhat.com>
+R:	Gurchetan Singh <gurchetansingh@chromium.org>
+R:	Chia-I Wu <olvaffe@gmail.com>
 L:	dri-devel@lists.freedesktop.org
 L:	virtualization@lists.linux-foundation.org
 S:	Maintained