BUG/MINOR: map: The map list was declared in the map.h file

This bug is harmless and post-dev19, it does not require any backport.
diff --git a/include/types/map.h b/include/types/map.h
index 15a0ef8..68f1a36 100644
--- a/include/types/map.h
+++ b/include/types/map.h
@@ -29,7 +29,7 @@
  *
  * "maps" is the list head. This list cotains all the mao file name identifier.
  */
-struct list maps = LIST_HEAD_INIT(maps); /* list of struct map_reference */
+extern struct list maps;
 
 struct map_reference {
 	struct list list;    /* used for listing */