Handle SYS_setns not existing but __NR_setns does.

On some older Fedora distros SYS_setns was not defined, but
__NR_setns is a usable replacement.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>

nsenter/nsexec.c

@@ -40,6 +40,9 @@ static int child_func(void *_arg)
 #if __GLIBC__ == 2 && __GLIBC_MINOR__ < 14
 #define _GNU_SOURCE
 #include "syscall.h"
+#if defined(__NR_setns) && !defined(SYS_setns)
+#define SYS_setns __NR_setns
+#endif
 #ifdef SYS_setns
 int setns(int fd, int nstype)
 {