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)
+        {
+
+        }
+    }
+}

osu.Game.Benchmarks/osu.Game.Benchmarks.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <OutputType>Exe</OutputType>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+</Project>