fonts: Fix compilation warning due to added fallback mechanism (servo#39803)

servo#39608 added a mechanism to read document language from layout. It was
used to make sure fallback font for Hiragana/Katakana/Kanji is same for
Japanese document.

Right now, the mechanism is only used for FreeType/macOS. For Windows,
it already did the job correctly even before servo#39608, so there is no need
for the mechanism yet.

Testing: Manually tested on https://ja.wikipedia.org/wiki/Servo

Signed-off-by: Euclid Ye <[email protected]>

Author: yezhizhen

components/fonts/lib.rs

@@ -43,6 +43,7 @@ pub(crate) enum EmojiPresentationPreference {
 pub struct FallbackFontSelectionOptions {
     pub(crate) character: char,
     pub(crate) presentation_preference: EmojiPresentationPreference,
+    #[cfg_attr(target_os = "windows", allow(dead_code))]
     pub(crate) lang: Option<String>,
 }