Use Tool 0 for Dual X Axis G29

See https://github.com/MarlinFirmware/Marlin/issues/5597#issuecomment-397683325

Marlin/src/gcode/bedlevel/abl/G29.cpp

@@ -268,6 +268,10 @@ void GcodeSuite::G29() {
    */
   if (!g29_in_progress) {
 
+    #if ENABLED(DUAL_X_CARRIAGE)
+      if (active_extruder != 0) tool_change(0);
+    #endif
+
     #if ENABLED(PROBE_MANUALLY) || ENABLED(AUTO_BED_LEVELING_LINEAR)
       abl_probe_index = -1;
     #endif