Migration to expect/actual: fix multiplatform integration tests

compiler/testData/multiplatform/incompatibleNestedClasses/common.kt

@@ -1,19 +1,19 @@
-header class O1 {
+expect class O1 {
     class N1
     interface N2
     object N3
 }
 
-header class O2 {
+expect class O2 {
     class N2
     inner class I2
 }
 
-header class O3 {
+expect class O3 {
     object Companion
     companion object Factory
 }
 
-header class O4 {
+expect class O4 {
     companion object
 }