Maintenance: Polish fqdncache.h #3472

Job	Run time
source-maintenance-tests	2m 59s
build-tests(ubuntu-22.04,gcc,default)	11m 39s
functionality-tests	17m 16s
CodeQL-tests	25m 1s
build-tests(ubuntu-22.04,gcc,minimal)	6m 19s
build-tests(ubuntu-22.04,gcc,maximus)	13m 24s
build-tests(ubuntu-22.04,clang,default)	15m 7s
build-tests(ubuntu-22.04,clang,minimal)	7m 22s
build-tests(ubuntu-22.04,clang,maximus)	15m 40s
build-tests(macos-14,clang,default)	14m 14s
build-tests(macos-14,clang,minimal)	10m 26s
build-tests(macos-14,clang,maximus)	13m 25s
	2h 32m 52s

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