V4L: Do not enable VIDEO_V4L2 unconditionally

The VIDEO_V4L2 config setting is enabled unconditionally, even for
configurations with no support for this subsystem whatsoever. The
following patch adds the necessary dependency.
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>