sqlite driver: avoid error when index exists already

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@56636 15284696-431f-4ddb-bdfa-cd5b030d7da7

db/drivers/sqlite/index.c

@@ -47,6 +47,7 @@ int db__driver_create_index(dbIndex * index)
 	db_append_string(&sql, " unique");
 
     db_append_string(&sql, " index ");
+    db_append_string(&sql, " if not exists ");
     db_append_string(&sql, db_get_index_name(index));
     db_append_string(&sql, " on ");