When HAVE_SETNS is not set, iproute2 provides a local implementation of this
function based on __NR_setns.
This macro is defined in sys/syscall.h, which was not included, thus the local
implementation always returned -1.

CC: Vadim Kochan <vadim4j@gmail.com>
Fixes: eb67e449 ("lib: Add netns_switch func for change network namespace")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>