(mingw) Fix MinGW build error for new LibXML2 headers #1145

Job	Run time
check-ubuntu-openssl300	2m 3s
check-ubuntu-openssl300 (--enable-size-t=yes)	2m 14s
check-ubuntu-openssl300 (--enable-static-linking=yes --without-nss --without-gcrypt --without-gnu...	59s
check-osx	4m 16s
check-mingw (MINGW64, x86_64, --enable-unicode=yes --enable-size-t=yes)	12m 13s
check-msvc (mscrypto, unicode=yes with-dl=yes)	1m 52s
check-osx (--enable-size-t=yes)	5m 28s
check-mingw (MINGW64, x86_64, --enable-unicode=yes --enable-size-t=no)	10m 2s
check-msvc (mscng, unicode=yes with-dl=yes)	2m 12s
check-ubuntu-openssl110	59s
check-mingw (MINGW64, x86_64, --enable-unicode=no --enable-size-t=no)	12m 51s
check-msvc (openssl-300, unicode=yes with-dl=yes)	4m 27s
check-ubuntu-openssl110 (--enable-size-t=yes)	59s
check-msvc (openssl-110, unicode=yes with-dl=yes)	2m 43s
check-ubuntu-openssl110 (--enable-static-linking=yes --without-nss --without-gcrypt --without-gnu...	52s
check-msvc (mscrypto, unicode=no with-dl=no)	1m 37s
check-msvc (mscng, unicode=no with-dl=no)	1m 53s
	1h 7m 40s

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