[media] e4000: make VIDEO_V4L2 dependency optional

That tuner driver is mainly for DVB API, but there is some V4L2 API
controls for SDR usage. Make driver compile conditional so that V4L2
is not mandatory. Without the V4L2 support driver is build as a DVB
only, without SDR controls.

Fixes following errors reported by kbuild test robot:
ERROR: "v4l2_ctrl_auto_cluster" [drivers/media/tuners/e4000.ko] undefined!
ERROR: "v4l2_ctrl_new_std" [drivers/media/tuners/e4000.ko] undefined!
ERROR: "v4l2_ctrl_handler_init_class" [drivers/media/tuners/e4000.ko] undefined!
ERROR: "v4l2_ctrl_handler_free" [drivers/media/tuners/e4000.ko] undefined!

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>