fix sign compare to make travis-ci happy

src/modules/mavlink/mavlink_messages.cpp

@@ -605,8 +605,8 @@ protected:
 			bat_msg.battery_function = MAV_BATTERY_FUNCTION_ALL;
 			bat_msg.type = MAV_BATTERY_TYPE_LIPO;
 			bat_msg.temperature = INT16_MAX;
-			for (unsigned i = 0; i < (sizeof(bat_msg.voltages) / sizeof(bat_msg.voltages[0])); i++) {
-				if (i < battery_status.cell_count) {
+			for (unsigned int i = 0; i < (sizeof(bat_msg.voltages) / sizeof(bat_msg.voltages[0])); i++) {
+				if ((int)i < battery_status.cell_count) {
 					bat_msg.voltages[i] = (battery_status.voltage_v / battery_status.cell_count) * 1000.0f;
 				} else {
 					bat_msg.voltages[i] = UINT16_MAX;