tests: posix: net: fix missing clock_t and clockid_t with newlib

Fix errors found in CI for missing clock_t and clockid_t types
when building the tests/posix/net testsuite against newlib.

Also use angle-brackets and prefixed path with
`<zephyr/posix/posix_types.h>` since it is not in the default
search path and is analagous to `<sys/types.h>`.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>