mavlink_ftp: handle relative paths correctly

[bkueng]

By ensuring there's a '/' in between when concatenating the path with _root_dir.

It turns out there's a bug in GQC where a component metadata uri like mftp://etc/extras/component_general.json.xz is turned into an ftp request with path /etc/extras/component_general.json.xz, whereas it should be etc/extras/component_general.json.xz.