libextra: unused import fix for android AGAIN

Author: yichoi

src/libextra/ebml.rs

@@ -97,11 +97,14 @@ pub mod reader {
     use core::cast::transmute;
     use core::int;
     use core::io;
+    use core::option::{None, Option, Some};
 
     #[cfg(target_arch = "x86")]
     #[cfg(target_arch = "x86_64")]
-    use core::option::{None, Option, Some};
     use core::ptr::offset;
+
+    #[cfg(target_arch = "x86")]
+    #[cfg(target_arch = "x86_64")]
     use core::unstable::intrinsics::bswap32;
 
     // ebml reading