Use an empty enum for the custom tag in the test

There's no point in allowing to have values of it, it's only a marker
type.

gstreamer/src/tags.rs

@@ -1014,7 +1014,7 @@ mod tests {
     fn test_custom_tags() {
         ::init().unwrap();
 
-        struct MyCustomTag;
+        enum MyCustomTag {};
 
         impl<'a> Tag<'a> for MyCustomTag {
             type TagType = &'a str;