network-manager-applet: add PACKAGCONFIG for modemmanager - disabled by default

by updating modemmanager configuration of network-manager-applet now auto
detects modemmanager - the old version had no (lib)mm-glib. By using a
packageconfig build is deterministic again.

Fixes:

| WARNING: QA Issue: network-manager-applet rdepends on modemmanager, but it isn't a build dependency? [build-deps]

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>