containers/netns: Add missed definition for check_netns_enabled.c

CLONE_NEWNET and CLONE_NEWNS are defined in bit/sched.h, and need __USE_GNU
defined first. Do the same as par_chld_ipv6.c and two_children_ns.c did, just
define _GNU_SOURCE directly.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>