settings.h: forward declare `AddonInfo` #19933

Job	Run time
build_cmake (ubuntu:24.04, true, true)	2m 11s
build_make (alpine:3.23)	6m 6s
build_make (ubuntu:25.10)	5m 3s
build_cmake (ubuntu:25.10, true, true)	2m 12s
build_make (ubuntu:24.04)	3m 58s
build_cmake (alpine:3.23, false, false)	4m 8s
	23m 38s

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