Remove an unused import.

I accidentally left it in after removing the calls to stdout/stderr in
d926b8342b.

ports/gonk/build.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use std::env;
-use std::process::{Command, Stdio};
+use std::process::Command;
 
 fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();