Support building all variants.

Bug: 143217452
Test: cd external/rust; mma; atest in all TEST_MAPPING subdirs
Change-Id: I8f1aab2880b18eb756cb050af7479ba97c4577f9
diff --git a/src/lib.rs b/src/lib.rs
index 529ee4a..91089e0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -40,6 +40,9 @@
 
 #![no_std]
 
+// ANDROID: Use std to allow building as a dylib.
+extern crate std;
+
 // ANDROID: count.rs generated and put in the out subdirectory.
 // include!(concat!(env!("OUT_DIR"), "/count.rs"));
 include!("../out/count.rs");