`struct timespec` is no longer available from headers after kernel 5.6.
It is now required to use `struct timespec64`, which uses a 64bit
integer for seconds instead of the former 32bit.

The function `timespec_to_ktime` becomes `timespec64_to_ktime`

The function `getnstimeofday` becomes `ktime_get_real_ts64`
Signed-off-by: Gwenhael GOAVEC <gwenhael.goavec@femto-st.fr>
Signed-off-by: Mathis MARION <mathis.marion@grenoble-inp.org>