[gegl] Fix newly introduced compile warning

[Øyvind Kolås <ok src gnome org>]

commit d6c16c86f323abac4602e1ce5c94f087c9d9e42a
Author: �yvind Kolås <pippin gimp org>
Date:   Thu Jan 13 22:31:15 2011 +0000

    Fix newly introduced compile warning
    
    size_t is not neccesarily an unsigend long, cast it so that it is so we get rid
    of the compiler warning.

 operations/external/png-load.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/operations/external/png-load.c b/operations/external/png-load.c
index c7a4c10..52aa1d8 100644
--- a/operations/external/png-load.c
+++ b/operations/external/png-load.c
@@ -58,7 +58,7 @@ static FILE * open_png(const gchar *path)
     {
       fclose(infile);
       g_warning ("%s is too short for a png file, only %lu bytes.",
-						  path, hdr_read_size);
+                 path, (unsigned long)hdr_read_size);
       return NULL;
     }