Fix compiler warning for old libxml2

Author: jeroen

src/xml2_init.c

@@ -4,8 +4,6 @@
 #include <libxml/parser.h>
 #include <string.h>
 
-static xmlExternalEntityLoader defaultLoader = NULL;
-
 /* * *
  * Author: Nick Wellnhofer <wellnhofer@aevum.de>
  * Date:   Tue, 24 Oct 2023 15:02:36 +0200
@@ -56,6 +54,8 @@ void handleGenericError(void *ctx, const char *fmt, ...){
 
 #if LIBXML_VERSION >= 21500
 
+static xmlExternalEntityLoader defaultLoader = NULL;
+
 xmlParserInput *download_file_callback(const char *url){
   SEXP arg = PROTECT(Rf_mkString(url));
   SEXP expr = PROTECT(Rf_install("download_file_callback"));