[msvc] fix for MSVC6 preprocessor not accepting blank parameters
[msvc] fix for MSVC6 preprocessor not accepting blank parameters

* unlike later iterations of Visual Studio, MSVC6 does not accept
  blank parameters on macro calls [eg. CALL(a, ,b)]
* blank params were used with the DLL_DECLARE and DLL_LOAD macros
* issue reported by Satz Klauer

pbatard/libusb-pbatard,pbatard/libusb-pbatard