Add 'rxjs' to plugins/package.json

Google-Bug-Id: b/237256547
Release-Notes: skip
Change-Id: I7c419b3d8c2277dba15899adb4cfef218f5aac13
diff --git a/plugins/package.json b/plugins/package.json
index b662e5b..82ebe43 100644
--- a/plugins/package.json
+++ b/plugins/package.json
@@ -6,7 +6,8 @@
     "@gerritcodereview/typescript-api": "3.6.0",
     "@polymer/decorators": "^3.0.0",
     "@polymer/polymer": "^3.4.1",
-    "lit": "^2.2.3"
+    "lit": "^2.2.3",
+    "rxjs": "^6.6.7"
   },
   "license": "Apache-2.0",
   "private": true