deps/opencv-3/main: Disable ovis CMake target for OpenCV
This is necessary to build OpenCV 3 under Ubuntu 18, as otherwise the error
deps/opencv-3/contrib/modules/ovis/src/ovis.cpp:7:10: fatal error: OgreApplicationContext.h: No such file or directory
❗ #include <OgreApplicationContext.h>
appears. This does not happen under U22, therefor the switch_os