Fixed the wrapping of free functions when using the single-file option.

Signed-off-by: Dimitar Dobrev <dpldobrev@yahoo.com>

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -214,7 +214,7 @@ namespace CppSharp.Generators.CSharp
             {
                 PushBlock(CSharpBlockKind.Functions);
                 WriteLine("public unsafe partial class {0}",
-                    TranslationUnits[0].FileNameWithoutExtension);
+                    context.TranslationUnit.FileNameWithoutExtension);
                 WriteStartBraceIndent();
 
                 PushBlock(CSharpBlockKind.InternalsClass);