Fix an overlooked issue I noticed with GL_KHR_debug.

With GLES3 context the function names must have a KHR suffix. This worked fine on K1 since it supports both GLES3 and GL.
On Mali this would fail since it is a GLES3 only device.
We didn't have to do this with the old GLExtensions namespace since it would grab the correct function pointers by name, but the function pointers
would end up under the same function pointer.