net_tstamp: define new flag HWTSTAMP_FLAG_BONDED_PHC_INDEX

As we defined the new hwtstamp_config flag HWTSTAMP_FLAG_BONDED_PHC_INDEX
as enum, it's not easy for userspace program to check if the flag is
supported when build.

Let's define the new flag so user space could build it on old kernel with
ifdef check.

Fixes: 9c9211a3 ("net_tstamp: add new flag HWTSTAMP_FLAG_BONDED_PHC_INDEX")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>