User subscriber instead of sender for var name in conn-mail-notif.c
User subscriber instead of sender for var name in conn-mail-notif.c

sender was confusing variable name since it is used has one of the mail
attributes in mail hash table.

Signed-off-by: Nicolas Dufresne <36c35e0aac84fcb59b447cc6415df42e7fda8df7@collabora.co.uk>

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