package/opencv: add a choice for selecting gstreamer support

OpenCV 3.0 support both gstreamer-0.10 and gstreamer-1.x, but only one
is used at the time.

This patch turns the gstreamer support into a choice, in order to prepare
adding the support for gstreamer-1 in a following patch.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>