Use static keyword on GlowGetProcAddress instead of unique suffix.

[dmitshur]

This is a simple cleanup task. It's a good first contribution for anyone interested in taking it.

In #87, GlowGetProcAddress was given a per-package unique suffix to fix go-gl/gl#85.

In go-gl/gl#85 (comment), @errcw realized that:

In retrospect it occurs to me that an equivalent fix might have been to mark the C functions as static (which, I realize now, is why the GL definitions do not experience the same conflict)

Since that approach is slightly simpler, we can update our code to use it.