libcob: Fix include for xmlCleanupParser

common.c uses xmlCleanupParser, which is defined in libxml/parser.h.
---

--- a/libcob/common.c	2025-01-01 03:03:49.762316279 +0100
+++ b/libcob/common.c	2025-01-01 03:01:56.632597306 +0100
@@ -136,6 +136,7 @@
 #if defined (WITH_XML2)
 #include <libxml/xmlversion.h>
 #include <libxml/xmlwriter.h>
+#include <libxml/parser.h>
 #endif
 
 #if defined (WITH_CJSON)