Add missing reference

For some reason premake didn't rebuild when I changed this and no compile error was thrown.

src/Generator.Tests/GeneratorTest.cs

@@ -2,6 +2,7 @@
 using System.IO;
 using System.Reflection;
 using System.Text.RegularExpressions;
+using System.Runtime.InteropServices;
 using CppSharp.AST;
 using CppSharp.Generators;
 using NUnit.Framework;