Fix Dual X FRS in M600

Co-Authored-By: InsanityAutomation <insanityautomation@users.noreply.github.com>

Marlin/src/gcode/feature/pause/M600.cpp

     int8_t DXC_ext = target_extruder;
     if (!parser.seen('T')) {  // If no tool index is specified, M600 was (probably) sent in response to filament runout.
                               // In this case, for duplicating modes set DXC_ext to the extruder that ran out.
-      #if ENABLED(FILAMENT_RUNOUT_SENSOR)
+      #if ENABLED(FILAMENT_RUNOUT_SENSOR) && NUM_RUNOUT_SENSORS > 1
         if (dxc_is_duplicating())
           DXC_ext = (READ(FIL_RUNOUT2_PIN) == FIL_RUNOUT_INVERTING) ? 1 : 0;
       #else