xfce4-pulseaudio-plugin: add packageconfig for libnotify

Add PACKAGECONFIG for libnotify and disable it by default
to avoid the QA issue:

WARNING: QA Issue: xfce4-pulseaudio-plugin rdepends on libnotify, but it isn't a build dependency? [build-deps]

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>