gl-renderer: Remove gl_renderer_interface from gl-renderer.h

gl-renderer: Remove gl_renderer_interface from gl-renderer.h

The rationale here is, that this line would create an instance of
gl_renderer_interface in every compilation unit that included
gl-renderer.h. This is not necessary, and it can actually be harmful by
masking the real exported gl_renderer_interface symbol, if you added
another compilation unit to gl-renderer.so, causing a runtime failure in
loading it.

gl-renderer.c already creates the exported symbol.