include/dri_interface.h: restore define mistakenly removed in !25587

include/dri_interface.h: restore define mistakenly removed in !25587

This file is a public API used by Xserver; removing something from it
means Xserver can't build anymore, and even if we fix Xserver, old
versions are still around and we want to keep them working to allow for
bisecting issues.

Fixes: 7301914755f5843f1095 ("dri: Remove __driDriverExtensions leftovers")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9976
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25692>