Linux: Add missing handling of tai field to __ntp_gettime64

This fixes a build error:

../sysdeps/unix/sysv/linux/ntp_gettime.c: In function ‘__ntp_gettime’:
../sysdeps/unix/sysv/linux/ntp_gettime.c:56:10: error: ‘ntv64.tai’ is used uninitialized in this function [-Werror=uninitialized]
   56 |   *ntv = valid_ntptimeval64_to_ntptimeval (ntv64);
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~