wifi: mac80211: fix maybe-unused warning

wifi: mac80211: fix maybe-unused warning

In ieee80211_lookup_key, the variable named `local` is unused if
compiled without lockdep, getting this warning:

net/mac80211/cfg.c: In function ‘ieee80211_lookup_key’:
net/mac80211/cfg.c:542:26: error: unused variable ‘local’ [-Werror=unused-variable]
  struct ieee80211_local *local = sdata->local;
                          ^~~~~

Fix it with __maybe_unused.

Fixes: 8cbf0c2ab6df ("wifi: mac80211: refactor some key code")
Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Link: https://lore.kernel.org/r/20221111153622.29016-1-ihuguet@redhat.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>