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
 }