Update Build.csproj

Author: dellis1972

build/Build.csproj

@@ -22,6 +22,7 @@
     <Using Include="Cake.Core.Diagnostics" />
     <Using Include="Cake.Core.IO" />
     <Using Include="Cake.FileHelpers" />
+    <Using Include="System.Runtime.InteropServices" />
   </ItemGroup>
 
   <ItemGroup>
@@ -34,4 +35,4 @@
     <ProjectReference Include="..\buildscripts\MonoGame.Tool.BuildScripts.csproj" />
   </ItemGroup>
 
-</Project>
+</Project>