Add empty benchmark project.

osu.Game.Benchmarks/Program.cs

@@ -0,0 +1,13 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Benchmarks
+{
+    public static class Program
+    {
+        public static void Main(string[] args)
+        {
+
+        }
+    }
+}