Add UNSOL_CALL_RINGBACK_TONE to ril.

Some networks may not play ring back tone, RIL will send UNSOL ringback notification.

It indicates to play tone to user for receiving ALERTING message.
diff --git a/libril/ril.cpp b/libril/ril.cpp
index f6f09cd..fc0a52a 100644
--- a/libril/ril.cpp
+++ b/libril/ril.cpp
@@ -3024,6 +3024,7 @@
         case RIL_UNSOL_CDMA_OTA_PROVISION_STATUS: return "UNSOL_CDMA_OTA_PROVISION_STATUS";
         case RIL_UNSOL_CDMA_INFO_REC: return "UNSOL_CDMA_INFO_REC";
         case RIL_UNSOL_OEM_HOOK_RAW: return "UNSOL_OEM_HOOK_RAW";
+        case RIL_UNSOL_RINGBACK_TONE: return "UNSOL_RINGBACK_TONE";
         default: return "<unknown request>";
     }
 }