media: platform: video-mux: fix Kconfig dependency

When CONFIG_V4L2 is built as a loadable module, the new video mux driver
fails to link as built-in code:

drivers/media/platform/video-mux.o: In function `video_mux_remove':
video-mux.c:(.text+0x24): undefined reference to `v4l2_async_unregister_subdev'
drivers/media/platform/video-mux.o: In function `video_mux_probe':
video-mux.c:(.text+0x800): undefined reference to `v4l2_subdev_init'
video-mux.c:(.text+0xa10): undefined reference to `v4l2_async_register_subdev'

This makes it use the same Kconfig dependency as all the other users of
the VIDEO_V4L2_SUBDEV_API symbol.

Fixes: 68803ad4 ("[media] platform: add video-multiplexer subdevice driver")

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>