include: Readd circular dependency between tree.h and parser.h

There are dozens of downstream projects that only include tree.h but use
declarations from parser.h. This broke after the recent cleanup of
circular dependencies.

Make tree.h include parser.h again. This is a hack but doesn't change
the include directory struture.

This commit only made it into the 2.12 branch but wasn't applied to
master, so the issue turned up in 2.13.0 again.

Should fix #734.

include/libxml/xmlIO.h

@@ -13,7 +13,9 @@
 #include <stdio.h>
 #include <libxml/xmlversion.h>
 #include <libxml/encoding.h>
+#define XML_TREE_INTERNALS
 #include <libxml/tree.h>
+#undef XML_TREE_INTERNALS
 
 #ifdef __cplusplus
 extern "C" {