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>
diff --git a/ggsn/gtp-kernel.h b/ggsn/gtp-kernel.h
index 628002f..83280a0 100644
--- a/ggsn/gtp-kernel.h
+++ b/ggsn/gtp-kernel.h
@@ -22,7 +22,7 @@
 				  struct in_addr *mask,
 				  struct gengetopt_args_info *args_info)
 {
-	if (args_info->gtpnl_given) {
+	if (args_info->gtp_linux_given) {
 		SYS_ERR(DGGSN, LOGL_ERROR, 0,
 			"ggsn compiled without GTP kernel support!\n");
 		return -1;