user/gitlab-foss: missing 'to_mb' function

Latest version of sys-filesystem fails to be used due to usage of statvfs64. Musl on 3.19 has removed that alias, thus sys-filesystem dies. As a fix, I applied https://github.com/jirutka/gitlab-foss/commit/15d47892b6a3fd6833fec7667f3a8281d5a123cc to remove the dependency, but apparently some functions were not reimplemented. Thus, gitlab has missing function errors.

A good fix would be to readd the dependency, maybe use v1.3.2 which uses statvfs on linux.