Fix SWIG_STD_VECTOR_ENHANCED macro used in C# std::vector to work with commas

git-svn-id: https://swig.svn.sourceforge.net/svnroot/swig/trunk@12081 626c5289-ae23-0410-ae9c-e8d60b6d4f22

Lib/csharp/csharp.swg

@@ -963,6 +963,9 @@ using System.Runtime.InteropServices;
 // Default enum handling
 %include <enums.swg>
 
+// For vararg handling in macros, from swigmacros.swg
+#define %arg(X...) X
+
 /*
 // Alternative char * typemaps.
 %pragma(csharp) imclasscode=%{