temp/xf86-video-opentegra: add a comment about removal (MR 4679)

[ci:skip-vercheck]: Changes don't affect the resulting package
[ci:skip-build]: Changes don't affect the resulting package

temp/xf86-video-opentegra/APKBUILD

+# Remove it when this MR is merged:
+# https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/57899
+
 # Maintainer: David Heidelberg <david@ixit.cz>
 pkgname=xf86-video-opentegra
 pkgver=0.6.0_git20211025