iwyu.yml: work around missing includes for Fedora [skip ci] #430

Job	Run time
iwyu (ubuntu-22.04, fedora:latest, libstdc++, -isystem/usr/lib/clang/20/include)	4m 27s
iwyu (macos-13, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	9m 37s
clang-include-cleaner (libc++)	7m 19s
clang-include-cleaner (libstdc++)	6m 6s
	27m 29s

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