[rilmodem][WIP] Removing compilation warning for ril_phonebook_init

Bugfix for removing compilation warning about not declaring
phonebook initialisation properly

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
[rilmodem][WIP] Introducing basic SMS cell broadcast support

Added handling of incoming SMS broadcast messages. Implementation
should enable device to receive emergency cell broadcast messages
but it needs more testing. Currently it has been tested only against
stub.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>