Try to fix an extern crate warning

Author: alexcrichton

src/lib.rs

@@ -58,7 +58,7 @@
 extern crate std;
 
 // This is only used for gimli right now, so silence warnings elsewhere.
-#[cfg_attr(not(target_os = "linux"), allow(unused_extern_crates))]
+#[cfg_attr(not(feature = "gimli-symbolize"), allow(unused_extern_crates))]
 extern crate alloc;
 
 pub use self::backtrace::{trace_unsynchronized, Frame};