Allow other processors to process JsonClass

Someone at KotlinConf brought this to my attention, as they're trying to generate their own adapter factory but can't since moshi consumes the annotation and doesn't allow others.

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessor.kt

@@ -90,7 +90,7 @@ class JsonClassCodegenProcessor : KotlinAbstractProcessor(), KotlinMetadataUtils
       }
     }
 
-    return true
+    return false
   }
 
   private fun adapterGenerator(element: Element): AdapterGenerator? {