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>
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