package/opencv: add gtk3 support

OpenCV now also supports gtk3 as a GUI toolkit, in addition to gtk2,
but only one may be enabled at a time.

So, add gtk3 in the choice to select the GUI toolkit.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: drop the superfluous depends-on for the
 kconfig symbol, since they're no longer needed now we depend-on rather
 than select]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>