Update acquire-time expiry check to use atomic.Int64 wrapper

The acquire-time lifetimeDestroyCount increment added in cc7ddf3 still
used the atomic.AddInt64 free function. PR #2563 converted the counter
fields to the atomic.Int64 typed wrapper, so update this call site to
match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jackc

pgxpool/pool.go

@@ -621,7 +621,7 @@ func (p *Pool) Acquire(ctx context.Context) (c *Conn, err error) {
 		// pinging) on them. This enforces MaxConnLifetime at acquire time so that
 		// a connection that expired while idle on a busy pool is not handed out.
 		if p.isExpired(res) {
-			atomic.AddInt64(&p.lifetimeDestroyCount, 1)
+			p.lifetimeDestroyCount.Add(1)
 			res.Destroy()
 			continue
 		}