compat: fix building clock_nanosleep function on OpenBSD

../compat/clock_nanosleep.c:46:9: error: use of undeclared identifier 'errno'
        errno = EINVAL;
        ^
../compat/clock_nanosleep.c:56:13: error: use of undeclared identifier 'errno'
            errno = EINVAL;
            ^
../compat/clock_nanosleep.c:94:9: error: use of undeclared identifier 'errno'
        errno = EINVAL;
        ^
3 errors generated.