media: rcar_drif: select CONFIG_V4L2_ASYNC

Without this, I see a randconfig link failure:

aarch64-linux-ld: drivers/media/platform/rcar_drif.o: in function `rcar_drif_remove':
rcar_drif.c:(.text+0x2a8): undefined reference to `v4l2_async_notifier_unregister'
aarch64-linux-ld: rcar_drif.c:(.text+0x2b0): undefined reference to `v4l2_async_notifier_cleanup'
aarch64-linux-ld: drivers/media/platform/rcar_drif.o: in function `rcar_drif_sdr_probe':
rcar_drif.c:(.text+0x1444): undefined reference to `v4l2_async_notifier_init'
aarch64-linux-ld: rcar_drif.c:(.text+0x14a0): undefined reference to `v4l2_async_notifier_register'
aarch64-linux-ld: rcar_drif.c:(.text+0x14d8): undefined reference to `v4l2_async_notifier_cleanup'
aarch64-linux-ld: rcar_drif.c:(.text+0x15a8): undefined reference to `__v4l2_async_notifier_add_fwnode_subdev'

I could not easily figure out when this was introduced, as this code
has not changed in a while but I only saw the problem recently.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/platform/Kconfig

@@ -638,6 +638,7 @@ config VIDEO_RCAR_DRIF
 	depends on VIDEO_V4L2
 	depends on ARCH_RENESAS || COMPILE_TEST
 	select VIDEOBUF2_VMALLOC
+	select V4L2_ASYNC
 	help
 	  Say Y if you want to enable R-Car Gen3 DRIF support. DRIF is Digital
 	  Radio Interface that interfaces with an RF front end chip. It is a