This will only be built when configured with P2P support. We can’t use
glib-mkenums here, as it doesn’t know about the #ifdef
FLATPAK_ENABLE_P2P which surrounds the enum definition. By manually
writing the get_type() function, we can surround it by #ifdefs as
appropriate.
Signed-off-by: Philip Withnall <withnall@endlessm.com>