iwyu.yml: work around missing includes for Fedora [skip ci] #426

Job	Run time
clang-include-cleaner (libstdc++)	4m 14s
iwyu (ubuntu-22.04, fedora:latest, libstdc++)	2m 51s
iwyu (ubuntu-22.04, fedora:latest, libc++, -isystem/usr/lib/clang/20/include)	4m 8s
clang-include-cleaner (libc++)	4m 14s
iwyu (macos-13, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	4m 22s
	19m 49s

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