GabbleJingleSession: wrap debug output in #ifdef ENABLE_DEBUG
GabbleJingleSession: wrap debug output in #ifdef ENABLE_DEBUG

This fixes an unused variable warning when disabling debug, e.g. in
distcheck.

community-ssu/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,jku/telepathy-gabble,community-ssu/telepathy-gabble,Ziemin/telepathy-gabble