lib: fix setns() function when !HAVE_SETNS

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>

include/namespace.h

@@ -3,6 +3,7 @@
 
 #include <sched.h>
 #include <sys/mount.h>
+#include <sys/syscall.h>
 #include <errno.h>
 
 #define NETNS_RUN_DIR "/var/run/netns"