plugins: Enable message_waiting in ril

Enable the message_waiting atom in the ril plugin.
This exposes the MessageWaiting interface which will
allow notification when new voicemails arrive.
src: fix sms_mwi_dcs_decode() return param bug

The return parameter 'active' was being incorrectly set to
an int value instead of a boolean value. This caused the
message-waiting code to break.
Merge pull request #47 from tonyespy/message-waiting

plugins: Enable message_waiting in ril