gtk2: Fix gcc 14 build. #488

Job	Run time
what-changed	25s
build-qemu (freebsd-14.2-x86_64, freebsd, x86-64, 14.2)	8m 2s
build-native (cygwin-2022-x86_64, windows-2022)	1h 23m 2s
build-native (ubuntu-24.04-x86_64, ubuntu-24.04)	4m 29s
build-native (macos-15-arm64, macos-15)	3m 8s
build-qemu (openbsd-7.6-x86_64, openbsd, x86-64, 7.6)	14m 27s
jenkins / trigger	11m 18s
build-native (macos-13-x86_64, macos-13)	1s
jenkins / build (centos-7-x86_64, https://ci.dreckly.dev/job/check-commit/check-commit=centos-7-x86_64/1112/)	2m 19s
jenkins / build (netbsd-10-x86_64, https://ci.dreckly.dev/job/check-commit/check-commit=netbsd-10-x86_64/11...	2m 16s
jenkins / build (smartos-24-x86_64, https://ci.dreckly.dev/job/check-commit/check-commit=smartos-24-x86_64/...	4m 35s
	2h 14m 2s

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