gst1-plugins-bad: add missing opencv ml module dependency

gst1-plugins-bad source code does include the old-style global opencv
header "opencv/cv.h", which includes headers from the core, imgproc,
video, features2d, flann, calib3d, objdetect and legacy modules.

However, the legacy module, in addition to headers from some of the
previously listed modules, also includes headers from the ml module.

Thus, the ml module must be selected to be able to build the legacy
one, so the gst-plugins-bad's opencv plugin.

Fixes:
  http://autobuild.buildroot.net/results/d02/d02e1843ef90f4b40c99e3b23eddf376f827d46e/



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