Update KotlinAnnotationIntrospector.kt (#616)

Fix using lazy init

Author: sarbyn

src/main/kotlin/com/fasterxml/jackson/module/kotlin/KotlinAnnotationIntrospector.kt

@@ -215,6 +215,6 @@ internal class KotlinAnnotationIntrospector(private val context: Module.SetupCon
     private fun KType.isRequired(): Boolean = !isMarkedNullable
 
     companion object {
-        val UNIT_TYPE: KType = Unit::class.createType()
+        val UNIT_TYPE: KType by lazy { Unit::class.createType() }
     }
 }