[Native] Add clean task to K/N:endorsedLibraries

Currently, :kotlin-native:endorsedLibraries does not have its build
directory deleted during a clean.

This task should probably be inherited from a plugin instead, but it is
not clear to me which one.

kotlin-native/endorsedLibraries/build.gradle.kts

@@ -91,3 +91,7 @@ tasks.register("endorsedLibsSources") {
         dependsOn("${library.taskName}NativeSources")
     }
 }
+
+tasks.register("clean") {
+    delete("build")
+}