Pass "--extern proc_macro" to rust_proc_macros.

This is actually required for some code.

Test: Modify the new test so it fails and see it fail.
Test: Build crates that fail without it.
Change-Id: I527752b765e5552aa2de7e201f056955e053e1f3

rust/Android.bp

@@ -50,6 +50,7 @@ bootstrap_go_package {
         "fuzz_test.go",
         "image_test.go",
         "library_test.go",
+        "proc_macro_test.go",
         "project_json_test.go",
         "protobuf_test.go",
         "rust_test.go",