Fix unused variable warning in GCC 16+ [GH edgcpp#198]

Author: ilazaric

src/util.h

@@ -4857,8 +4857,8 @@ formatter::append_into functions.
     total_size += element_sizes[i];
   }  /* for */
 
-  auto   *backing_array = reserve_func(total_size);
-  size_t counter = 0;
+  auto                *backing_array = reserve_func(total_size);
+  LOCAL_UNUSED size_t counter = 0;
   /* The following expression is expanded to effectively evaluate as:
 
        detail::String_formatter<type_1>::append_into(*backing_array,