[plasma-mobile] add modemmanager-pp to pinephone installs

ModemManager stuff has started to go into the kde-unstable repo
But since the stable packages have not changed yet,
we can't really switch to ModemManager quite yet.

So instead, just install the modemmanager-pp package,
so it's easier for developers to test.
This way they only have to disable ofono.service
and enable modemmanager.service to test it.

Signed-off-by: Dan Johansen <strit@manjaro.org>