map: add scale_cb for map module

Summary:
- in elm_map's _source_mod_cb(), scale_cb is needed to
  load MAP tile module.
- currently, test_map mod.c lacks this scale_cb
  and fails to load Map Tile source.

Reviewers: woohyun, Hermet, cedric

Reviewed By: cedric

Differential Revision: https://phab.enlightenment.org/D3702

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>

legacy/elementary/src/modules/test_map/mod.c

@@ -62,6 +62,12 @@ map_module_tile_coord_to_geo(const Evas_Object *obj EINA_UNUSED, int zoom EINA_U
    return EINA_FALSE;
 }
 
+EAPI double
+map_module_tile_scale_get(const Evas_Object *obj EINA_UNUSED, double lon EINA_UNUSED, double lat EINA_UNUSED, int zoom EINA_UNUSED)
+{
+   return 0;
+}
+
 static Eina_Bool
 _module_init(void)
 {