Add HAL_SERVO_LIB to HAL_Due.h

Marlin/src/HAL/HAL_DUE/HAL_Due.h

@@ -96,6 +96,8 @@
 
 typedef int8_t pin_t;
 
+#define HAL_SERVO_LIB Servo
+
 // --------------------------------------------------------------------------
 // Public Variables
 // --------------------------------------------------------------------------