xml BUGFIX handling namespaces in parent/child elements of the same name

correctly remove active namespaces (via lyxml_ns_rm()) in case the
parent element holding the namespace definition has the same name
(stored as the same memory chunk) as its child.
diff --git a/src/xml.h b/src/xml.h
index e12af7d..70431ea 100644
--- a/src/xml.h
+++ b/src/xml.h
@@ -48,6 +48,7 @@
     const char *element;  /* element where the namespace is defined */
     char *prefix;         /* prefix of the namespace, NULL for the default namespace */
     char *uri;            /* namespace URI */
+    unsigned int element_depth; /* depth level of the element to distinguish parent-child elements of the same name */
 };
 
 /* element tag identifier for matching opening and closing tags */