Fixes problem with IK classes not found

Just manually added the files to the solution. Fixes #295

spine-csharp/spine-csharp.csproj

@@ -69,6 +69,8 @@
     <Compile Include="src\AnimationState.cs" />
     <Compile Include="src\Event.cs" />
     <Compile Include="src\EventData.cs" />
+    <Compile Include="src\IkConstraint.cs" />
+    <Compile Include="src\IkConstraintData.cs" />
     <Compile Include="src\Json.cs" />
     <Compile Include="src\Skeleton.cs" />
     <Compile Include="src\SkeletonData.cs" />