message: add cancelled state

Based on patch from Yang Gu <gyagp0@gmail.com>

src/message.c

@@ -46,6 +46,8 @@ static const char *message_state_to_string(enum message_state s)
 		return "sent";
 	case MESSAGE_STATE_FAILED:
 		return "failed";
+	case MESSAGE_STATE_CANCELLED:
+		return "cancelled";
 	}
 
 	return NULL;

src/message.h

@@ -24,7 +24,8 @@
 enum message_state {
 	MESSAGE_STATE_PENDING,
 	MESSAGE_STATE_SENT,
-	MESSAGE_STATE_FAILED
+	MESSAGE_STATE_FAILED,
+	MESSAGE_STATE_CANCELLED,
 };
 
 struct ofono_atom;