openrisc: remove unnecessary stddef.h include

This causes the build to fail when building with the or1k-musl-linux-
toolchain and it is not needed.
Signed-off-by: Stafford Horne <shorne@gmail.com>

arch/openrisc/kernel/ptrace.c

@@ -16,7 +16,6 @@
  *      2 of the License, or (at your option) any later version.
  */
 
-#include <stddef.h>
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/string.h>