load_extension was referenced unconditionally, but is only defined

in extended.c if SLAPD_EXTERNAL_EXTENSIONS is defined. Add #ifdef here.

servers/slapd/module.c

@@ -14,10 +14,12 @@ struct module_regtable_t {
 	char *type;
 	int (*proc)(const void *module, const char *file_name);
 } module_regtable[] = {
-							{ "null", load_null },
-							{ "extension", load_extension },
-							{ NULL, NULL }
-						};
+		{ "null", load_null },
+#ifdef SLAPD_EXTERNAL_EXTENSIONS
+		{ "extension", load_extension },
+#endif
+		{ NULL, NULL }
+};
 
 typedef struct module_loaded_t {
 	struct module_loaded_t *next;