iwlwifi: Don't compile for gcc before 14

gcc 13 and earlier don't have __builtin_bitcountg. The linux wifi kpi
uses this unconditionally. While in this one use, it might not be
needed, I opted to not compile iwlwifi when building gcc12 or 13 rather
than risk breaking it for everbody else.

With this change gcc12 builds the kernel. Maybe this will stop jenkins
email for every commit I make.

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