[babl] export-symbols: add babl_backtrack for gimp-2.8

[Øyvind Kolås <ok src gnome org>]

commit 1d38f419e439aaeba2a8d9b2ee8c4f4292f7c0b8
Author: Øyvind Kolås <pippin gimp org>
Date:   Thu Oct 5 23:12:57 2017 +0200

    export-symbols: add babl_backtrack for gimp-2.8

 export-symbols |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/export-symbols b/export-symbols
index 28b3104..2ba4bb8 100644
--- a/export-symbols
+++ b/export-symbols
@@ -66,3 +66,4 @@ babl_type_is_symmetric
 babl_model_is_symmetric
 babl_fish_db
 babl_polynomial_approximate_gamma
+babl_backtrack