Fix homing Z position

Add `sync_plan_position()` after `axis_is_at_home(axis)` to keep the
planner position in sync when homing.

Marlin/Marlin_main.cpp

 
     // Set the axis position to its home position (plus home offsets)
     axis_is_at_home(axis);
+    sync_plan_position();
 
     destination[axis] = current_position[axis];
     feedrate = 0.0;