Fix misuse of enums, and hence compilation with gcc-4.5
Fix misuse of enums, and hence compilation with gcc-4.5

TP_OBSERVE_CHANNELS_CONTEXT_STATE_NONE == 0 and
TP_HANDLE_CHANNELS_CONTEXT_STATE_NONE == 0, so this typo was harmless.

Reviewed-by: Guillaume Desmottes <f16649c7afcd84e3350287183c4ed23cb1aea57c@collabora.co.uk>

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib