media: v4l: constify v4l2_ioctls[]

v4l2_ioctls[] is never modified, so mark it 'const'.
This way it gets placed in .rodata.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>