style/font: don't warn on dead code

The second String is not used anywhere so the Rust compiler
will complain:

warning: field `1` is never read
  -->
  /home/adi/workspace/web/deps/plotters/plotters/src/style/font/web.rs:21:37
     |
  21 | pub struct FontDataInternal(String, String);
     |            ----------------         ^^^^^^
     |            field in this struct
     |
     = note: `FontDataInternal` has a derived impl for the trait `Clone`, but this is intentionally ignored during dead code analysis
     = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
     |
  21 | pub struct FontDataInternal(String, ());
     |                                     ~~

warning: `plotters` (lib) generated 1 warning

plotters/src/style/font/web.rs

@@ -18,6 +18,7 @@ impl std::fmt::Display for FontError {
 impl std::error::Error for FontError {}
 
 #[derive(Clone)]
+#[allow(dead_code)]
 pub struct FontDataInternal(String, String);
 
 impl FontData for FontDataInternal {