Use SWIG-specific for non-overloaded synthesized functions too

This avoids conflicts between such functions, which are generated when
using %extend to add static methods to an existing class, and the actual
wrapper functions generated by the backend.

This shouldn't result in any user-visible changes.

Source/Modules/lang.cxx

@@ -1321,8 +1321,11 @@ int Language::staticmemberfunctionHandler(Node *n) {
     mrename = mangled;
 
     if (code) {
+      // See Swig_MethodToFunction() for the explanation of this code.
       if (Getattr(n, "sym:overloaded")) {
 	Append(cname, Getattr(defaultargs ? defaultargs : n, "sym:overname"));
+      } else {
+	Append(cname, "__SWIG");
       }
 
       if (!defaultargs) {