[babl] tools: follow constness change in icc parsing error handling

[Øyvind Kolås <ok src gnome org>]

commit ee81d5bfa3d9d9f0527cfb75ce9628bff3448742
Author: Øyvind Kolås <pippin gimp org>
Date:   Sun Sep 3 15:09:57 2017 +0200

    tools: follow constness change in icc parsing error handling

 tools/babl-icc-rewrite.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/tools/babl-icc-rewrite.c b/tools/babl-icc-rewrite.c
index af9d46e..1d292b4 100644
--- a/tools/babl-icc-rewrite.c
+++ b/tools/babl-icc-rewrite.c
@@ -35,7 +35,7 @@ main (int    argc,
   char *icc_data = NULL;
   long  icc_len;
   int genlen;
-  char *error = NULL;
+  const char *error;
   babl_init ();
 
   if (!argv[1] || !argv[2])