xdgmime: configure as submodule
So that we can bind it to a specific revision, and help with keeping clones up to date.
GitLab CI instructs to use a relative path to submodules on the same GitLab server [0], but doing so causes a meson/ninja dist failure [1]. Configure using a full https URL for now, as it appears to work with GitLab just fine.
[0] https://docs.gitlab.com/ce/ci/git_submodules.html [1] https://github.com/mesonbuild/meson/issues/8144