Fix missing dependency from postprocessing package

Author: BenGraterUnity

com.unity.postprocessing/package.json

@@ -6,6 +6,7 @@
     "unityRelease": "19f1",
     "description": "The post-processing stack (v2) comes with a collection of effects and image filters you can apply to your cameras to improve the visuals of your games.",
     "dependencies": {
-      "com.unity.modules.physics": "1.0.0"
+      "com.unity.modules.physics": "1.0.0",
+      "com.unity.modules.imgui": "1.0.0"
     }
 }