[build][fidl] Fix //zircon/public/fidl metadata

Step #1's local testing was misconfigured and failed to test the
step #2 integration build properly as the bots do, so a trivial
bug slipped through.

Bug: ZX-3365
Change-Id: I331b3b989125c5982632d310daa8b7d512a49faf

zircon/public/gn/fidl.gni

@@ -286,6 +286,8 @@ template("fidl_library") {
           legacy_dirs += [ "fidl/$fidl_name" ]
           legacy_targets += [
             {
+              _label = get_label_info(":$target_name", "label_with_toolchain")
+              _zircon_public = "fidl"
               import = "//build/fidl/fidl.gni"
               target_type = "fidl"
               target_name = fidl_name