add expect/actual opt in option

Author: Mr3zee

grpc/grpc-core/build.gradle.kts

@@ -8,6 +8,10 @@ plugins {
 }
 
 kotlin {
+    compilerOptions {
+        freeCompilerArgs.add("-Xexpect-actual-classes")
+    }
+
     sourceSets {
         commonMain {
             dependencies {