sd-bus-vtable: add dummy macro to support compile without GNU extension

sd-bus-vtable: add dummy macro to support compile without GNU extension

If SD_BUS_METHOD_WITH_ARGS() is set with SD_BUS_NO_ARGS and/or SD_BUS_NO_RESULT,
then it introduces
_SD_VARARGS_FOREACH_EVEN(_SD_ECHO, NULL)
    -> _SD_VARARGS_FOREACH_SEQ(_01, …, _50, NULL)
Hence, the variadic argument `...` in _SD_VARARGS_FOREACH_SEQ() has no
argument, but it is not allowed if built without GNU extension, e.g. -std=c11.

Let's introduce one more unused dummy argument to support such situation.