while COMPILE_TEST is y and MAILBOX is n, build fails:

ERROR: modpost: "mbox_request_channel_byname" [drivers/media/platform/amphion/amphion-vpu.ko] undefined!
ERROR: modpost: "mbox_free_channel" [drivers/media/platform/amphion/amphion-vpu.ko] undefined!
ERROR: modpost: "mbox_send_message" [drivers/media/platform/amphion/amphion-vpu.ko] undefined!

Adding a MAILBOX dependency in VIDEO_AMPHION_VPU to fix this.

Link: https://lore.kernel.org/linux-media/20220318030305.39120-1-yuehaibing@huawei.com
Fixes: 52b3a219 ("media: platform: amphion: move config to its own file")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>