Add direct reference to System.IO.Compression

Classic Net461 apps require this even when referencing NetStandard1.6
when consuming a NetStandard library consuming ImageSharp.... Wut?!

src/ImageSharp/ImageSharp.csproj

@@ -41,6 +41,7 @@
     <PackageReference Include="System.Threading.Tasks.Parallel" Version="4.3.0" />
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.3.0" />
     <PackageReference Include="System.Numerics.Vectors" Version="4.3.0" />
+    <PackageReference Include="System.IO.Compression" Version="4.3.0" />
   </ItemGroup>
   <PropertyGroup>
     <CodeAnalysisRuleSet>..\..\ImageSharp.ruleset</CodeAnalysisRuleSet>