Migrate header to expect in libraries projects

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/multiplatformProject/lib/src/main/kotlin/PlatformClass.kt

@@ -1,6 +1,6 @@
 package foo
 
-header class PlatformClass {
+expect class PlatformClass {
     val value: String
 }
 

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/multiplatformProject/lib/src/test/kotlin/PlatformTest.kt

@@ -1,6 +1,6 @@
 package foo
 
-header class PlatformTest {
+expect class PlatformTest {
     val value: PlatformClass
 }
 

libraries/tools/kotlin-maven-plugin-test/src/it/test-multiplatform/shared/src/main/kotlin/org/jetbrains/api.kt

@@ -1,6 +1,6 @@
 package org.jetbrains
 
-header fun doMain()
+expect fun doMain()
 
 fun getGreeting() : String {
     return "Hello, World!"