Make digitalPotWrite depend on HAS_DIGIPOTSS

Marlin/stepper.h

@@ -62,7 +62,9 @@ extern block_t* current_block;  // A pointer to the block currently being traced
 
 void quickStop();
 
-void digitalPotWrite(int address, int value);
+#if HAS_DIGIPOTSS
+  void digitalPotWrite(int address, int value);
+#endif
 void microstep_ms(uint8_t driver, int8_t ms1, int8_t ms2);
 void microstep_mode(uint8_t driver, uint8_t stepping);
 void digipot_init();