BUG/MINOR: diag: always show the version before dumping a diag warning

BUG/MINOR: diag: always show the version before dumping a diag warning

Diag warnings were added in 2.4 by commit 7b01a8dbd ("MINOR: global:
define diagnostic mode of execution") but probably due to the split
function that checks for the mode, they did not reuse the emission of
the version string before the first warning, as was brought in 2.2 by
commit bebd21206 ("MINOR: init: report in "haproxy -c" whether there
were warnings or not"). The effet is that diag warnings are emitted
before the version string if there is no other warning nor error. Let's
just proceed like for the two other ones.

This can be backported to 2.4, though this is of very low importance.