package/gstreamer1/gst1-plugins-bad: fix deactivation of opencv

Build can fail if opencv3 is built before gst1-plugins-bad because
-Dopencv=disabled does not work in meson (i.e. since commit
5d6c408e)

Fixes:
 - http://autobuild.buildroot.org/results/19605057c4956d97e9e65068680485db637282db



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>