egl: bring back the default glapi.so name

egl: bring back the default glapi.so name

Earlier commit replaced the default platform specific libglapi.so name
with an #error.

This may have been overzealous since the name is the correct for the BSD
platforms, at least. Reinstate the hunk - bringing back OpenBSD, et al.
to a successful build state.

Fixes: 7a9c92d071d ("egl/dri2: non-shared glapi cleanups")
[Emil Velikov: format the patch from Eric, add commit message and tag.]
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>