package: ruby-prometheus-client-mmap

I hope to update rust-nix to 0.30 soon. ruby-prometheus-client-mmap needs
a small patch to build with the new version.

Unfortunately, the small patch breaks builds with the old version, so
uploading this change to sid will have to wait until the new rust-nix is
actuallly in sid, in the meantime the new rust-nix is available in
experimetal
for testing with.