Add amphp/sync to deps

Author: trowski

composer.json

@@ -27,6 +27,7 @@
         "amphp/amp": "^3",
         "amphp/parallel": "^2.2.3",
         "amphp/serialization": "^1",
+        "amphp/sync": "^2.1",
         "laravel/serializable-closure": "^1.3"
     },
     "require-dev": {