linux-user/syscall.c: define _ATFILE_SOURCE

Needed to make sure the xxxat() functions are available.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NRiku Voipio <riku.voipio@iki.fi>

linux-user/syscall.c

@@ -18,6 +18,7 @@
  *  Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
  *  MA 02110-1301, USA.
  */
+#define _ATFILE_SOURCE
 #include <stdlib.h>
 #include <stdio.h>
 #include <stdarg.h>