Support building all variants.

Bug: 143217452
Test: cd external/rust; mma; atest in all TEST_MAPPING subdirs
Change-Id: I675ad5a4215baaffc99d4b4a1b5072c470b107c4
diff --git a/src/lib.rs b/src/lib.rs
index 1ee35c8..68e5b7e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -49,7 +49,7 @@
 #![cfg_attr(feature = "bench", feature(test))]
 #![no_std]
 
-#[cfg(test)]
+// ANDROID: Unconditionally use std to allow building as a dylib.
 #[macro_use]
 extern crate std;