Fix #167 incompatible pointer types warnings

[simleb]

This fixes the warnings. I hope it doesn't break anything else.

[dmitshur]

I think this LGTM. But I have no way of testing this since I don't use this repo.

If someone else can test this, let's merge it since it's a minor warning fix.

Otherwise, let's close this since this repo is now in legacy status. It's recommended to update to better, current and maintained set of bindings at https://github.com/go-gl/gl.

[dmitshur]

http://stackoverflow.com/questions/26419420/define-gldebugmessagecallbackarb-callback-method may be relevant.