Try fix jenkins windows builds

I have no idea whether that would help, but considering that jenkins has problem with:
```
/usr/lib64/gcc/i686-w64-mingw32/4.8.2/../../../../i686-w64-mingw32/bin/ld: cannot find -llibopencv_core249
/usr/lib64/gcc/i686-w64-mingw32/4.8.2/../../../../i686-w64-mingw32/bin/ld: cannot find -llibopencv_highgui249
/usr/lib64/gcc/i686-w64-mingw32/4.8.2/../../../../i686-w64-mingw32/bin/ld: cannot find -llibopencv_imgproc249
```
and there's: https://stackoverflow.com/questions/14914830/linking-opencv-a-libraries-with-mingw-and-qt-5-0/14927151#14927151

I guess it's worth a try.