XMSS Wconversion fixes #6398

Job	Run time
build library (--disable-asm --enable-all-crypto --enable-mlkem --enable-lms --enable-xmss --enab...	1m 3s
build library (--disable-asm --enable-mlkem --enable-lms --enable-xmss --enable-mldsa)	40s
build library (--enable-intelasm --enable-sp-asm --enable-all-crypto --enable-mlkem --enable-lms ...	52s
build library (--disable-asm)	33s
build library (--enable-intelasm --enable-sp-asm --enable-mlkem --enable-lms --enable-xmss --enab...	35s
build library (--enable-intelasm --enable-sp-asm)	34s
	4m 17s

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