iwlwifi: Disable -Wformat when building with GCC.

GCC's -Wformat complains about NULL format strings passed to
iwl_fw_dbg_collect_trig (though the function handles NULL format
strings). Curious that upstream iwlwifi in Linux is built with GCC
and explicitly opts into this warning via the __printf() attribute.

Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D34146