Save some PROGMEM with constexpr (#11798)

When possible, make `active_extruder` a `constexpr` to save some PROGMEM.

Marlin/src/module/planner.h

@@ -101,7 +101,9 @@ typedef struct {
   };
   uint32_t step_event_count;                // The number of step events required to complete this block
 
-  uint8_t active_extruder;                  // The extruder to move (if E move)
+  #if EXTRUDERS > 1
+    uint8_t active_extruder;                // The extruder to move (if E move)
+  #endif
 
   #if ENABLED(MIXING_EXTRUDER)
     uint32_t mix_steps[MIXING_STEPPERS];    // Scaled steps[E_AXIS] for the mixing steppers