libnamespaces: fix warning about syscall()

The warning was:
In file included from namespace.c:14:0:
../include/namespace.h: In function ‘setns’:
../include/namespace.h:37:2: warning: implicit declaration of function ‘syscall’ [-Wimplicit-function-declaration]
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>

include/namespace.h

@@ -3,6 +3,7 @@
 
 #include <sched.h>
 #include <sys/mount.h>
+#include <unistd.h>
 #include <sys/syscall.h>
 #include <errno.h>