opencv: highgui module is needed for ts module

OpenCV's highgui module needs to be auto-selected when the ts module is
selected, because it depends on it. Otherwise it would fail with an
error like this one:

[ 96%] Building CXX object
modules/ts/CMakeFiles/opencv_ts.dir/src/gpu_perf.cpp.o
In file included from
/home/test/autobuild/instance-1/output/build/opencv-2.4.8/modules/ts/src/gpu_perf.cpp:43:0:
/home/test/autobuild/instance-1/output/build/opencv-2.4.8/modules/ts/include/opencv2/ts/gpu_perf.hpp:47:39:
fatal error: opencv2/highgui/highgui.hpp: No such file or directory
compilation terminated.

Fixes:
  http://autobuild.buildroot.net/results/31b/31b124ff1ec958a621863bfc80323847a6373135/

Issue reported upstream:
  http://code.opencv.org/issues/3923



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>