zdtm: Fix building for @_BSD_SOURCE redefinition

Travis testing engine complained about @_BSD_SOURCE
which is not the case on my build environment.
Anyway lets do a safe thing -- conditional definition.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/lib/fs.h

 #ifndef ZDTM_FS_H_
 #define ZDTM_FS_H_
 
-#define _BSD_SOURCE
+#ifndef _BSD_SOURCE
+# define _BSD_SOURCE
+#endif
+
 #include <sys/types.h>
 
 #include <limits.h>