Support building all variants.

Bug: 143217452
Test: cd external/rust; mma; atest in all TEST_MAPPING subdirs
Change-Id: I29d8288db0e4569fd737eee69488693e7f4352cb
diff --git a/Android.bp b/Android.bp
index c8253df..909135a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -5,7 +5,7 @@
     crate_name: "proc_macro_hack",
     srcs: ["src/lib.rs"],
     edition: "2018",
-    rlibs: [
+    rustlibs: [
         "libproc_macro2",
         "libquote",
         "libsyn",