TpPresenceMixin: remove references to an empty priv structure
TpPresenceMixin: remove references to an empty priv structure

Patch from Sunil Mohan Adapa <sunil@synovel.com>: MSVC++ doesn't
allow empty structures, which are a gcc extension.

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