Add common constructor to ViewModel class

Author: rickclephas

kmp-observableviewmodel-properties/src/commonMain/kotlin/com/rickclephas/kmp/observableviewmodel/properties/ViewModel.kt

@@ -5,4 +5,4 @@ import com.rickclephas.kmp.observableviewmodel.ViewModel
 /**
  * A Kotlin Multiplatform ViewModel.
  */
-public expect abstract class ViewModel: ViewModel
+public expect abstract class ViewModel(): ViewModel