Fix: realsense2-gl headers not installed
Fix: realsense2-gl headers not installed

This allows `librealsense2` and `librealsense2-gl` to be used when
installed (e.g. `/usr/local/`) by adding the following to
`CMakeLists.txt`.

```
find_package(realsense2 REQUIRED)
find_package(realsense2-gl REQUIRED)
```

IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense,IntelRealSense/librealsense