iwlwifi: mei: fix building iwlmei

Building iwlmei without CONFIG_CFG80211 causes a link-time warning:

ld.lld: error: undefined symbol: ieee80211_hdrlen
>>> referenced by net.c
>>>               net/wireless/intel/iwlwifi/mei/net.o:(iwl_mei_tx_copy_to_csme) in archive drivers/built-in.a

Add an explicit dependency to avoid this. In theory it should not
be needed here, but it also seems pointless to allow IWLMEI
for configurations without CFG80211.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Emmanuel Grumbach <Emmanuel.grumbach@intel.com>
Acked-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220316183617.1470631-1-arnd@kernel.org