block/iocost: silence warning on 'last_period' potentially being unused

If CONFIG_TRACEPOINTS isn't enabled, we assign this variable but then
never use it. This can cause the compiler to complain about that:

block/blk-iocost.c:1264:6: warning: variable 'last_period' set but not used [-Wunused-but-set-variable]
 1264 |         u64 last_period, cur_period;
      |             ^

Rather than add ifdefs to guard this, just mark it __maybe_unused.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401102335.GiWdeIo9-lkp@intel.com/Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -1261,7 +1261,7 @@ static void weight_updated(struct ioc_gq *iocg, struct ioc_now *now)
 static bool iocg_activate(struct ioc_gq *iocg, struct ioc_now *now)
 {
 	struct ioc *ioc = iocg->ioc;
-	u64 last_period, cur_period;
+	u64 __maybe_unused last_period, cur_period;
 	u64 vtime, vtarget;
 	int i;