www/firefox: try to fix build on diverse CPUs

the cpuid.h header errors out on anythinbg non-x86, as per
http://ampere1.nyi.freebsd.org/data/123arm64-quarterly/5889de908603/logs/firefox-99.0_1,2.log
limit the include to x86 - the actual use of that was already limited
to amd64, everything else gets the fallthrough

(cherry picked from commit 4a4ffa51d5f0f57bd4047b04fe348594a89fdd0d)