pidfd_send_signal: Build with _GNU_SOURCE

This adds missing O_CLOEXEC and O_DIRECTORY on old distros,
where it's (in glibc) guarded under __USE_GNU.

This was needed on SLES 11 (glibc 2.11).

Acked-by: Jan Stancek <[email protected]>
Signed-off-by: Petr Vorel <[email protected]>

Author: pevik

testcases/kernel/syscalls/pidfd_send_signal/pidfd_send_signal01.c

@@ -9,6 +9,7 @@
  * struct is passed.
  */
 
+#define _GNU_SOURCE
 #include <signal.h>
 #include <stdlib.h>
 #include "tst_safe_pthread.h"

testcases/kernel/syscalls/pidfd_send_signal/pidfd_send_signal02.c

@@ -21,6 +21,7 @@
  *    -> EPERM
  */
 
+#define _GNU_SOURCE
 #include <pwd.h>
 #include <signal.h>
 #include "tst_safe_pthread.h"

testcases/kernel/syscalls/pidfd_send_signal/pidfd_send_signal03.c

@@ -15,6 +15,7 @@
  * child using the pidfd of the first one.
  */
 
+#define _GNU_SOURCE
 #include <signal.h>
 #include <stdio.h>
 #include <unistd.h>