Stephane Bidoul pointed out that it doesn't define IN_LIBXML Daniel

* chvalid.c genChRanges.py: Stephane Bidoul pointed out that
  it doesn't define IN_LIBXML
Daniel
diff --git a/genChRanges.py b/genChRanges.py
index 4962a81..d4b6018 100755
--- a/genChRanges.py
+++ b/genChRanges.py
@@ -274,6 +274,8 @@
  * William Brack <wbrack@mmm.com.hk>
  */
 
+#define IN_LIBXML
+#include "libxml.h"
 #include <libxml/chvalid.h>
 
 /*