Move Stepper::synchronize to Planner (#10713)

Co-Authored-By: ejtagle <ejtagle@hotmail.com>

Marlin/src/feature/snmm.cpp

@@ -27,7 +27,7 @@
 #include "../module/stepper.h"
 
 void select_multiplexed_stepper(const uint8_t e) {
-  stepper.synchronize();
+  planner.synchronize();
   disable_e_steppers();
   WRITE(E_MUX0_PIN, TEST(e, 0) ? HIGH : LOW);
   WRITE(E_MUX1_PIN, TEST(e, 1) ? HIGH : LOW);