build: memfd -- Use SYS_memfd_create for libc sake

Just got a situation inside VM where pretty new
kernel with memfd has been installed (and as result
__NR_memfd_create shipped with kernel headers
is provided as well) but libc was old having no
SYS_memfd_create defined. Thus we've got an error
because we use exactly SYS_ number for calls.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/feature-tests.mak

@@ -100,7 +100,7 @@ define MEMFD_TEST
 
 int main(void)
 {
-#ifdef __NR_memfd_create
+#ifdef SYS_memfd_create
 	return 0;
 #else
 #error No memfd support