AP_Mount: Conditionally define serial_instance to fix unused variable compile error

- Wrapped the definition of `serial_instance` with preprocessor directives to ensure it is only defined when necessary.
- This resolves the compile error caused by the unused variable when no features requiring `serial_instance` are enabled.

libraries/AP_Mount/AP_Mount.cpp

@@ -184,6 +184,8 @@ void AP_Mount::init()
             set_mode_to_default(instance);
         }
     }
+
+    (void)serial_instance;
 }
 
 // update - give mount opportunity to update servos.  should be called at 10hz or higher