(mingw) Fix MinGW build error for new LibXML2 headers #1140

Job	Run time
check-osx	4m 35s
check-mingw (MINGW64, x86_64, --enable-unicode=yes --enable-size-t=yes)	6m 4s
check-osx (--enable-size-t=yes)	6m 11s
check-ubuntu-openssl110	57s
check-msvc (mscrypto, unicode=yes with-dl=yes)	1m 47s
check-mingw (MINGW64, x86_64, --enable-unicode=yes --enable-size-t=no)	4m 52s
check-ubuntu-openssl110 (--enable-size-t=yes)	57s
check-mingw (MINGW64, x86_64, --enable-unicode=no --enable-size-t=no)	4m 12s
check-msvc (mscng, unicode=yes with-dl=yes)	2m 10s
check-ubuntu-openssl110 (--enable-static-linking=yes --without-nss --without-gcrypt --without-gnu...	51s
check-ubuntu-openssl300	1m 58s
check-ubuntu-openssl300 (--enable-size-t=yes)	2m 13s
check-msvc (openssl-300, unicode=yes with-dl=yes)	4m 24s
check-ubuntu-openssl300 (--enable-static-linking=yes --without-nss --without-gcrypt --without-gnu...	56s
check-msvc (openssl-110, unicode=yes with-dl=yes)	2m 29s
check-msvc (mscrypto, unicode=no with-dl=no)	1m 50s
check-msvc (mscng, unicode=no with-dl=no)	2m 5s
	48m 31s

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