Fix unused function warning in non-p2p case
Fix unused function warning in non-p2p case

_flatpak_dir_fetch_remote_state_metadata_branch is only used when building
with p2p, so #ifdef the entire function.

Closes: #1575
Approved by: alexlarsson

flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak