Cleanup: rename BKE_animdata_{add=>ensure}_id

Use the term `ensure` as existing data is used when present.

source/blender/makesrna/intern/rna_ID.c

@@ -772,7 +772,7 @@ static struct ID *rna_ID_make_local(struct ID *self, Main *bmain, bool clear_pro
 
 static AnimData *rna_ID_animation_data_create(ID *id, Main *bmain)
 {
-  AnimData *adt = BKE_animdata_add_id(id);
+  AnimData *adt = BKE_animdata_ensure_id(id);
   DEG_relations_tag_update(bmain);
   return adt;
 }