gpui: Gate Linux screen capture APIs behind feature flag (#50300)

We were missing the cfg statements in the `LinuxClient` trait definition

Release Notes:

- N/A

crates/gpui_linux/src/linux/platform.rs

@@ -55,12 +55,12 @@ pub(crate) trait LinuxClient {
     fn display(&self, id: DisplayId) -> Option<Rc<dyn PlatformDisplay>>;
     fn primary_display(&self) -> Option<Rc<dyn PlatformDisplay>>;
 
-    #[allow(dead_code)]
+    #[cfg(feature = "screen-capture")]
     fn is_screen_capture_supported(&self) -> bool {
         false
     }
 
-    #[allow(dead_code)]
+    #[cfg(feature = "screen-capture")]
     fn screen_capture_sources(
         &self,
     ) -> oneshot::Receiver<Result<Vec<Rc<dyn gpui::ScreenCaptureSource>>>> {