salut-connection: including avahi header only when we use USE_BACKEND_AVAHI
salut-connection: including avahi header only when we use USE_BACKEND_AVAHI

Including the file without any condtions results in a compilation error

freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut