posix: fix posix clock undefined type

When built with the minimal libc while using POSIX clocks, Zephyr
compilation breaks with "error: unknown type name 'clockid_t'." Use
int instead of clockid_t to eliminate the dependency.

Suggested-by: Chris Friedt <cfriedt@tenstorrent.com>
Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>