Make 'add' function abstract in AbstractMutableSet in JVM

libraries/stdlib/src/kotlin/collections/AbstractMutableSet.kt

@@ -5,6 +5,5 @@ import java.util.AbstractSet
 
 @SinceKotlin("1.1")
 public abstract class AbstractMutableSet<E> protected constructor() : MutableSet<E>, AbstractSet<E>() {
-    // nothing to make abstract
-    // it's a class rather than typealias in order to have bridge for `size` generated and nice non-platform types in methods
+    abstract override fun add(element: E): Boolean
 }