Add missing Ds\Set::reduce to stubs.

Author: ZebulanStanphill

stubs/ext-ds.stub

@@ -700,6 +700,16 @@ final class Set implements Collection, ArrayAccess
 	{
 	}
 
+	/**
+	 * @template TCarry
+	 * @param callable(TCarry, TValue): TCarry $callback
+	 * @param TCarry $initial
+	 * @return TCarry
+	 */
+	public function reduce(callable $callback, $initial = null)
+	{
+	}
+
 	/**
 	 * @param TValue ...$values
 	 */