fix linux build errors (OgreOverlay)

Hi, I'm using Arch Linux and wanted to build this project, which failed, but this fixed it.
I'm not really clear how your branches are supposed to work, but I think this pullrequest affects both. 

Is this supposed to run on Linux at the moment? While I can compile it, it instantly crashes on startup. If it is, I'll create another bugreport on that with additional information, if it isn't… I guess I wont.