package/opencv: fix dependencies for the Qt and GTK backends

Only the highgui module can use Qt or GTK (as noticed by Thomas.)

However, it is currently possible to select the Qt or the GTK backends
support independently of highgui, thus breaking the configuration step
of opencv.

Just hide the Qt and GTK backends when highgui is not selected.

Fixes:
    http://autobuild.buildroot.org/results/efc/efc9590a193235701fee535f4a1da7b39c426665/
    http://autobuild.buildroot.org/results/b6a/b6acd983b8c78005feef6864af469bd348ab742a/



[Thomas: remove 'if BR2_PACKAGE_OPENCV_LIB_HIGHGUI' statements since
BR2_PACKAGE_OPENCV_WITH_QT now depends on
BR2_PACKAGE_OPENCV_LIB_HIGHGUI]

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