fix compilation error when RYML_NO_DEFAULT_CALLBACKS is defined #1099

Job	Run time
samples (Release, ubuntu-24.04)	4m 10s
samples (Debug, ubuntu-24.04)	2m 41s
samples (Release, macos-latest)	2m 21s
samples (Debug, macos-latest)	2m 6s
samples (Release, windows-2022)	9m 36s
samples (Debug, windows-2022)	2m 56s
	23m 50s

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