Use 'friend' to access set_directions

Marlin/src/module/stepper.h

@@ -466,10 +466,14 @@ class Stepper {
       #endif
     }
 
+  private:
+
     // Set direction bits for all steppers
     static void set_directions();
 
-  private:
+    // Allow reset_stepper_drivers to access private set_directions
+    friend void reset_stepper_drivers();
+
     // Set the current position in steps
     static void _set_position(const int32_t &a, const int32_t &b, const int32_t &c, const int32_t &e);