Fix build if EXTRUDERS = 0 (#15417)

Marlin/src/module/motion.cpp

   const uint16_t old_pct = feedrate_percentage;
   feedrate_percentage = 100;
 
-  const float old_fac = planner.e_factor[active_extruder];
-  planner.e_factor[active_extruder] = 1.0f;
+  #if EXTRUDERS
+     const float old_fac = planner.e_factor[active_extruder];
+     planner.e_factor[active_extruder] = 1.0f;
+  #endif
 
   #if IS_KINEMATIC
     if (is_fast)
 
   feedrate_mm_s = old_feedrate;
   feedrate_percentage = old_pct;
-  planner.e_factor[active_extruder] = old_fac;
+  #if EXTRUDERS
+    planner.e_factor[active_extruder] = old_fac;
+  #endif
 }
 
 /**