Remove relative gl_common.h include directives

I've started to work on a PPSSPP core for [OpenEmu](https://github.com/OpenEmu/OpenEmu) over here: https://github.com/PGGB/PPSSPP-Core

As we don't use GLEW I use a [modified gl_common.h](https://github.com/PGGB/PPSSPP-Core/blob/master/gfx/gl_common.h), so we can have the PPSSPP source simply as a submodule. The two files I modified in this pull request are the exception to this as they use a relative include directive.

Thanks a lot for the amazing work you've been doing. I hope we will be able to provide a great way to use PPSSPP on OS X.