This uses some definitions from <sys/cdefs.h> in gcc 8.4.0, not present
in musl or gcc11.

Also use clock_gettime() instead of syscall(__NR_clock_gettime,...),
which is not currently defined.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

Signed-off-by: Jan Pavlinec <jan.pavlinec@nic.cz>