Add a central list of native libs needed by 3p Rust crates.
This allows rust_* targets in the tree to use such crates while still using a vendored directory (i.e. non-GNified) for 3p crates.
diff --git a/libraries.gni b/libraries.gni
new file mode 100644
@@ -0,0 +1,16 @@
+# Copyright 2017 The Fuchsia Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+# Keeps track of crates requiring a native library. These lists should be kept
+# up-to-date when vendored crates are updated.
+# We define these in a central location so that we don't need to add BUILD.gn
+# files for individual crates and rely fully on a vendor directory.
+rust_native_libs_names = [
+rust_native_libs_targets = [