python/types.c: Fix building against older libxml2

The XML_IGNORE_FPTR_CAST_WARNINGS and XML_POP_WARNINGS macros were added very
recently, so define them as nothing if they aren't defined upon including the
libxml2 headers.

python/types.c

@@ -29,6 +29,13 @@
 
 #ifdef _WIN32
 
+#ifndef XML_IGNORE_FPTR_CAST_WARNINGS
+#define XML_IGNORE_FPTR_CAST_WARNINGS
+#endif
+#ifndef XML_POP_WARNINGS
+#define XML_POP_WARNINGS
+#endif
+
 #include <windows.h>
 #include <crtdbg.h>