You might try the mesa libs but most of the drivers that are in non free should install opengl.The nvidia installer, mind you, their installer and not xbps, has args to install the headers. You may be able to tell xbps to pass those arguments to the installer, though I have a feeling it's packaged and not going through the normal steps.
In any event, most gfx cards will have their own headers for you to use, otherwise they can be found here: https://www.opengl.org/
registry/
You may also want to query xbps for GLUT.