media: i2c: ths8200 needs V4L2_ASYNC

Fix the build errors reported by the kernel test robot by
selecting V4L2_ASYNC:

mips-linux-ld: drivers/media/i2c/ths8200.o: in function `ths8200_remove':
ths8200.c:(.text+0x1ec): undefined reference to `v4l2_async_unregister_subdev'
mips-linux-ld: drivers/media/i2c/ths8200.o: in function `ths8200_probe':
ths8200.c:(.text+0x404): undefined reference to `v4l2_async_register_subdev'

Fixes: ed29f894 ("media: i2c: ths8200: support asynchronous probing")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Lad Prabhakar <prabhakar.csengg@gmail.com>
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>