iwlwifi: adjust to make d3.c compile

struct iwl_mvm_wep_key_cmd ends in a variable sized array but later
is included in another struct followed by the actual
struct iwl_mvm_wep_key in d3.c. Make the array[0] instead of []
to avoid the warning about it being a GNU-extension.
Also include string.h explicitly for memset_after().

Reviewed by: cc
Differential Revision: https://reviews.freebsd.org/D43647

(cherry picked from commit e1d31d0685f0b430f385023b7de49f47be6c7de0)