Remove typedef for fips_glXGetProcAddressARB_t

Remove typedef for fips_glXGetProcAddressARB_t

There were two problems with this typedef. First, we don't actually
need it, (we define it here and then use it exactly once on the next
line---it's simpler to have the direct syntax for a function returning
a pointer to a function accepting void and returning void.

More importantly, the typedef was relying on the type __GLXextFuncPtr
being defined. This happens to work with Mesa on my system but is
inherently fragile. So the code is more robust not relying on this.