Add cmake rules to install mmal headerfiles to /opt/vc/include/interface...

These patches to the CMakeLists.txt files in interface/mmal* also install the mmal header files to include/interface/mmal/*. They're needed to link to lib/mmal*.so.

Tested on a native build, hence the /opt/vc prefix in the commit message.

Maybe related: http://www.raspberrypi.org/phpBB3/viewtopic.php?t=45328&p=358808