Add app-check-interop-types to externs.json.

Author: ehsannas

packages/firestore/externs.json

@@ -14,6 +14,7 @@
     "packages/app-types/index.d.ts",
     "packages/app-types/private.d.ts",
     "packages/app/dist/app.d.ts",
+    "packages/app-check-interop-types/index.d.ts",
     "packages/auth-interop-types/index.d.ts",
     "packages/firestore/dist/lite/internal.d.ts",
     "packages/firestore/dist/internal.d.ts",