[babl] export-symbols: add babl_format_exists

[Øyvind Kolås <ok src gnome org>]

commit a4ef3078f5d6ff37052fcbd8b1b8b39a60555bbf
Author: Øyvind Kolås <pippin gimp org>
Date:   Tue Oct 31 20:48:23 2017 +0100

    export-symbols: add babl_format_exists

 export-symbols |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/export-symbols b/export-symbols
index 2ba4bb8..f85fec9 100644
--- a/export-symbols
+++ b/export-symbols
@@ -8,6 +8,7 @@ babl_exit
 babl_fast_fish
 babl_fish
 babl_format
+babl_format_exists
 babl_format_get_bytes_per_pixel
 babl_format_get_model
 babl_format_get_n_components