Update stepper.cpp

Forgot Z2_MIN_PIN

Marlin/stepper.cpp

       WRITE(Z_MIN_PIN,HIGH);
     #endif
   #endif
+  
+  #if HAS_Z2_MIN
+    SET_INPUT(Z2_MIN_PIN);
+    #if ENABLED(ENDSTOPPULLUP_ZMIN)
+      WRITE(Z2_MIN_PIN,HIGH);
+    #endif
+  #endif
 
   #if HAS_X_MAX
     SET_INPUT(X_MAX_PIN);