Restrict visibility to existing users.

Test: m rust
Change-Id: I0b5af970209b50f1113be8b76c1ef41b74766a1e
diff --git a/Android.bp b/Android.bp
index 8818cea..c5c8eae 100644
--- a/Android.bp
+++ b/Android.bp
@@ -64,4 +64,8 @@
     ],
     vendor_available: true,
     product_available: true,
+    visibility: [
+        "//external/rust/crates/x509-parser",
+        "//external/rust/crates/oid-registry",
+    ],
 }