bgpd: Increment version number even when no data is sent

bgpd: Increment version number even when no data is sent

When an update group decides to not send a prefix
announcement because it has not changed, still increment
the version number.  Why?  To allow for the situation
where you have say 2 peers in 1 peer group and shortly
after they come up a 3rd peer comes up.  It will be
placed into a separate update group and could be
coalesced down, when it finishes updating all data
to it.  Now imagine that a single prefix changes at
this point in time as well.  Then first 2 peers may
decide to not send the data, since nothing has changed.
While the 3rd peer will and since the versions numbers
never match they will never coalesce.  So when the decision
is made to skip, update the version number as well.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
(cherry picked from commit a8038460bd2b413a0c082fbd629cdae00708b133)