Fix build with libxml >=2.14.

Index: src/tools.c
--- src/tools.c.orig
+++ src/tools.c
@@ -293,7 +293,7 @@ validate_xml(GtkWindow *parent, char *text) {
 		g_string_free(xml_error_context_hack, TRUE);
 		return FALSE;
 	}
-	initGenericErrorDefaultFunc(NULL);
+	xmlSetGenericErrorFunc(NULL, NULL);
 	xmlFreeParserCtxt(ctxt);
 	g_string_free(xml_error_context_hack, TRUE);
 	return TRUE;
