gst1-plugins-bad: add missing depends on C++ for the OpenCV plugin

OpenCV requires C++ support, so we shouldn't select it without
depending on C++ support.

Fixes:
http://autobuild.buildroot.org/results/71f/71f2d8a267a0d159521407a9a2330e68d756c4b3/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>