gst1-plugins-bad: fix missing dependency for opencv plugin

Gstreamer1 opencv plugin requires opencv's contrib module.

Fixes:
  http://autobuild.buildroot.net/results/0aa/0aab79be8043bf88a227af2ca27b4447aa2f8901/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>