tests/memfd_create.c: use libtests

* tests/memfd_create.c: Use SKIP_MAIN_UNDEFINED.
diff --git a/tests/memfd_create.c b/tests/memfd_create.c
index 98c3064..9fe7f6d 100644
--- a/tests/memfd_create.c
+++ b/tests/memfd_create.c
@@ -13,10 +13,6 @@
 
 #else
 
-int
-main(void)
-{
-	return 77;
-}
+SKIP_MAIN_UNDEFINED("__NR_memfd_create")
 
 #endif