Fixed build break in mutex6.c

Build break was introduced as part of changes done for
upstream.

Signed-off-by: Himanshu Chauhan <hschauhan@nulltrace.org>

tests/mutex6.c

@@ -27,14 +27,6 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef STAND_ALONE
-#include <stddef.h>
-#else
-#include <printk.h>
-#include <atom-types.h>
-#endif
-
-
 #include "atom.h"
 #include "atomtests.h"
 #include "atommutex.h"