[gimp] app: fix a build warning.

[Jehan <jehanp src gnome org>]

commit 73b960aefb3d5b7d965b80930eb367f6d3c874e7
Author: Jehan <jehan girinstud io>
Date:   Mon Jul 20 15:41:22 2020 +0200

    app: fix a build warning.
    
    Fixes:
    > warning: passing argument 1 of ‘GIMP_CONFIG’ discards ‘const’
    > qualifier from pointer target type.

 app/core/gimpsymmetry.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/app/core/gimpsymmetry.c b/app/core/gimpsymmetry.c
index 0d1ac16c35..1b9e1cb44a 100644
--- a/app/core/gimpsymmetry.c
+++ b/app/core/gimpsymmetry.c
@@ -504,7 +504,7 @@ gimp_symmetry_to_parasite (const GimpSymmetry *sym)
 
   parasite_name = gimp_symmetry_parasite_name (G_TYPE_FROM_INSTANCE (sym));
 
-  parasite = gimp_config_serialize_to_parasite (GIMP_CONFIG (sym),
+  parasite = gimp_config_serialize_to_parasite ((GimpConfig *) sym,
                                                 parasite_name,
                                                 GIMP_PARASITE_PERSISTENT,
                                                 NULL);