selftests/clone3: Fix compiler warning

selftests/clone3: Fix compiler warning

Shuah reported a compiler warning with an Ubuntu GCC 13 build, I've been
unable to reproduce it but hopefully this fixes the issue:

clone3_set_tid.c:136:43: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t’ {aka ‘long unsigned int’} [-Wformat=]

Reported-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>