notifier: Fix compile warning
Fixes ../src/calls-notifier.c: In function ‘state_changed_cb’: /usr/include/glib-2.0/glib/gmacros.h:1132:17: warning: ‘notification’ may be used uninitialized in this function [-Wmaybe-uninitialized]
which wasn't problematic with the current code flow but can easily become a problem when shuffling stuff around.