Use raw::c_void for GLvoid.
Use raw::c_void for GLvoid.

074e3193f changed almost all void pointer types to use raw::c_void type,
except GLvoid. It looks like an accidental omission, since they both
where changed to () in c0a29ece and were libc::c_void before that.

skligys/gl-rs,bjz/gl-rs,skligys/gl-rs,bjz/gl-rs