v4l: Export v4l2-common.h in include/linux/Kbuild

v4l2-common.h is a header file that's used in user space, thus it must be
exported using header-y.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>