[gegl] fix build without mrg

[Øyvind Kolås <ok src gnome org>]

commit 05ff191c14f92cc586ba6871212c73e805eacde4
Author: Øyvind Kolås <pippin gimp org>
Date:   Fri Jul 14 21:21:02 2017 +0200

    fix build without mrg

 gcut/gcut.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gcut/gcut.c b/gcut/gcut.c
index fedcd61..d25bb06 100644
--- a/gcut/gcut.c
+++ b/gcut/gcut.c
@@ -1190,6 +1190,7 @@ int gcut_ui_main (GeglEDL *edl);
 int gcut_ui_main (GeglEDL *edl);
 int gcut_ui_main (GeglEDL *edl)
 {
+  fprintf (stderr, "gcut built without mrg UI\n");
   return -1;
 }
 #endif