[functions.reinterpret] fix warning

dune/xt/functions/reinterpret.hh

@@ -98,7 +98,7 @@ private:
     {
     }
 
-    virtual size_t order(const Common::Parameter& mu = {}) const override final
+    virtual size_t order(const Common::Parameter& /*mu*/ = {}) const override final
     {
       return order_;
     }