media: i2c: select V4L2_ASYNC where needed

I came across a link failure from randconfig builds:

x86_64-linux-ld: drivers/media/i2c/ths8200.o: in function `ths8200_remove':
ths8200.c:(.text+0x491): undefined reference to `v4l2_async_unregister_subdev'
x86_64-linux-ld: drivers/media/i2c/ths8200.o: in function `ths8200_probe':
ths8200.c:(.text+0xe49): undefined reference to `v4l2_async_register_subdev'
x86_64-linux-ld: drivers/media/i2c/tw9910.o: in function `tw9910_remove':
tw9910.c:(.text+0x467): undefined reference to `v4l2_async_unregister_subdev'
x86_64-linux-ld: drivers/media/i2c/tw9910.o: in function `tw9910_probe':
tw9910.c:(.text+0x1123): undefined reference to `v4l2_async_register_subdev'

These clearly lack a 'select' statement, but I don't know why
this started happening only now. I had a bit of a look around to find
other configs that have the same problem, but could not come up with
a reliable way and found nothing else through experimentation.
It is likely that other symbols like these exist that need an extra
select.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>