Add base64 externs for ReactNative build

Author: schmidt-sebastian

packages/firestore/externs.json

@@ -22,6 +22,7 @@
     "packages/logger/dist/src/logger.d.ts",
     "packages/webchannel-wrapper/src/index.d.ts",
     "packages/util/dist/src/environment.d.ts",
+    "packages/util/dist/src/crypt.d.ts",
     "packages/firestore/src/protos/firestore_proto_api.d.ts",
     "packages/firestore/src/util/error.ts",
     "packages/firestore/src/local/indexeddb_schema.ts",