config-store: forward declare xmlTextWriter

Signed-off-by: Gabriel Ferreira <gabrielcarvfer@gmail.com>

src/config-store/model/xml-config.cc

@@ -29,6 +29,7 @@
 #include "ns3/string.h"
 
 #include <libxml/encoding.h>
+#include <libxml/xmlreader.h>
 #include <libxml/xmlwriter.h>
 
 namespace ns3

src/config-store/model/xml-config.h

@@ -22,10 +22,13 @@
 
 #include "file-config.h"
 
-#include <libxml/xmlreader.h>
-#include <libxml/xmlwriter.h>
 #include <string>
 
+// These are in #include <libxml/xmlwriter.h>,
+// here we just need a forward declaration.
+typedef struct _xmlTextWriter xmlTextWriter;
+typedef xmlTextWriter* xmlTextWriterPtr;
+
 namespace ns3
 {