misc: Fix 'unused variable' clang errors with gem5.fast

Change-Id: I2bb8ac10e8db69fa82abe41577cd8e5db575e93d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70297
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/dev/storage/ide_disk.cc

@@ -1072,7 +1072,7 @@ IdeDisk::serialize(CheckpointOut &cp) const
     Tick reschedule = 0;
     Events_t event = None;
 
-    int eventCount = 0;
+    [[maybe_unused]] int eventCount = 0;
 
     if (dmaTransferEvent.scheduled()) {
         reschedule = dmaTransferEvent.when();