selftests/sched_ext: Remove duplicated unistd.h include in rt_stall.c

The header <unistd.h> is included twice in rt_stall.c. Remove the
redundant inclusion to clean up the code.

Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: EricccTaiwan

tools/testing/selftests/sched_ext/rt_stall.c

@@ -15,7 +15,6 @@
 #include <signal.h>
 #include <bpf/bpf.h>
 #include <scx/common.h>
-#include <unistd.h>
 #include "rt_stall.bpf.skel.h"
 #include "scx_test.h"
 #include "../kselftest.h"