media: i2c: ov2640: Depend on V4L2_ASYNC

Add V4L2_ASYNC as a dependency to match other drivers and prevent failures
when compile testing.

Fixes: ff3cc65c ("media: v4l: async, fwnode: Improve module organisation")
Signed-off-by: Mike Pagano <mpagano@gentoo.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>