Fix for https://github.com/glfw/glfw/issues/222

Fix for a compile time error on Linux.

Since Jul 28, original [GLFW master branch includes `Xrandr.h` by default](https://github.com/glfw/glfw/commit/036da0), so this issue can be solved either by adding the header to [Arturo's fork](https://github.com/arturoc/glfw/tree/feature-keysUnicode) or here, in openFrameworks.