XMLPUBVAR: patch the mingw block, not the cygwin

In the patch that is now in MXE, somehow the line numbers got
messed up. The result is that the XMLPUBVAR is now fixed for
the cygwin case, not for mingw. This patch fixes that. It adds
'extern' to the __declspec(dllexport) definition of XMLPUBVAR.

This is also what is already in upstream libxml2, including in the
version that is shipped with mxe. See:
1eabc31401

src/vtk-1-fixes.patch

@@ -64,7 +64,7 @@ diff --git a/ThirdParty/libxml2/vtklibxml2/include/libxml/xmlexports.h b/ThirdPa
 index 1111111..2222222 100644
 --- a/ThirdParty/libxml2/vtklibxml2/include/libxml/xmlexports.h
 +++ b/ThirdParty/libxml2/vtklibxml2/include/libxml/xmlexports.h
-@@ -135,7 +135,7 @@
+@@ -111,7 +111,7 @@
    #undef XMLCDECL
    #if defined(IN_LIBXML) && !defined(LIBXML_STATIC)
      #define XMLPUBFUN __declspec(dllexport)