selftests/sigaltstack: Fix wrong format specifier

Compiling sigaltstack selftest after adding a __printf() attribute to
ksft_print_msg() exposes -Wformat warning in main().
The format specifier inside ksft_print_msg() expects a long
unsigned int but the passed variable is of unsigned int type.

Fix the format specifier so it matches the passed variable.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>