Update header file name for Kotlin 1.4.20

Tracking JetBrains/kotlin@7c3eda3 to start support for Kotlin 1.4.20 and newer.

Author: JakeWharton

gradle-plugin/src/main/kotlin/co/touchlab/kotlin/gradle/targets/native/tasks/CocoapodsTasks.kt

@@ -197,7 +197,7 @@ open class DummyFrameworkTask : DefaultTask() {
         // Copy files for the dummy framework.
         copyFrameworkFile("Info.plist")
         copyFrameworkFile("dummy", frameworkName)
-        copyFrameworkFile("Headers/dummy.h")
+        copyFrameworkFile("Headers/placeholder.h")
         copyFrameworkTextFile("Modules/module.modulemap") {
             if (it == "framework module dummy {") {
                 it.replace("dummy", frameworkName)