iwyu.yml: work around missing includes for Fedora [skip ci] #424

Job	Run time
iwyu (ubuntu-22.04, fedora:latest, libstdc++)	4m 40s
iwyu (ubuntu-22.04, fedora:latest, libc++)	5m 19s
iwyu (macos-13, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	3m 28s
clang-include-cleaner	0s
	13m 27s

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