Export rpc_unstable header

It'll be used by microdroid payload.

Test: build
Change-Id: I6eaccb55f291a7426ee047bbf6f1f2d211b5b94d
diff --git a/libs/binder/rust/Android.bp b/libs/binder/rust/Android.bp
index fe2da18..d9d7caf 100644
--- a/libs/binder/rust/Android.bp
+++ b/libs/binder/rust/Android.bp
@@ -109,7 +109,7 @@
 // TODO(b/184872979): remove once the Rust API is created.
 rust_bindgen {
     name: "libbinder_rpc_unstable_bindgen",
-    wrapper_src: "src/binder_rpc_unstable.hpp",
+    wrapper_src: ":libbinder_rpc_unstable_header",
     crate_name: "binder_rpc_unstable_bindgen",
     source_stem: "bindings",
     shared_libs: [