Redo this commit, and remove the inclusion of gl2ext.h
from src/mapi/glapi/glapi_priv.h.  The include was added in
8f3be339

 to fix a missing prototype for
glDrawBuffersNV and others, but it's not possible to include both
glext.h and gl2ext.h from the same file.

I don't see the missing prototype here (with or without shared glapi)
so I'm just removing the offending #include.

Also, since we're redoing this, update to the most recent gl2ext.2.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>