ggsn: fix compilation error with --disable-gtp-kernel

gtp-kernel.h: In function ‘gtp_kernel_init’:
gtp-kernel.h:25:15: error: ‘struct gengetopt_args_info’ has no member named ‘gtpnl_given’
  if (args_info->gtpnl_given) {
               ^
Makefile:422: recipe for target 'ggsn

Reported-by: Holger Freyther <holger@freyther.de>
1 file changed