fixes compile failure:
logging.c: In function '_gpgrt_log_printhex':
logging.c:1153:49: error: incompatible type for argument 4 of '_gpgrt_logv_printhex'
     _gpgrt_logv_printhex (buffer, length, NULL, NULL);
                                                 ^~~~
logging.c:1097:1: note: expected 'va_list {aka __va_list}' but argument is of type 'void *'
 _gpgrt_logv_printhex (const void *buffer, size_t length,
 ^~~~~~~~~~~~~~~~~~~~