Remove hard dependency of GLX from Dolphin with X11.

This is particularly annoying on a EGL only system that doesn't have GLX.
Remove a hard dependency from libGL from the software renderer.

I must have missed this when dropping OpenGL library includes. This is annoying when on a system that doesn't have libGL.
Merge pull request #1340 from Sonicadvance1/EGL-fixes

Remove hard dependencies of GLX and libGL from Dolphin.