selftests/mm: remove duplicate include of unistd.h

Remove duplicate inclusion of unistd.h in memory-failure.c to clean up
redundant code.

Link: https://lkml.kernel.org/r/20260211064311.2981726-1-nichen@iscas.ac.cn
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Acked-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/memory-failure.c

@@ -13,7 +13,6 @@
 #include <unistd.h>
 #include <signal.h>
 #include <setjmp.h>
-#include <unistd.h>
 #include <fcntl.h>
 #include <sys/vfs.h>
 #include <linux/magic.h>