[vte] sixel: Add missing ifdef

[Christian Persch <chpe src gnome org>]

commit 4a9629531ac5ae59fe0c87f46a64788691295ad4
Author: Christian Persch <chpe src gnome org>
Date:   Sat Feb 13 21:31:08 2021 +0100

    sixel: Add missing ifdef

 src/vteinternal.hh | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/src/vteinternal.hh b/src/vteinternal.hh
index c4788412..41766c0e 100644
--- a/src/vteinternal.hh
+++ b/src/vteinternal.hh
@@ -293,7 +293,9 @@ public:
                 /* ECMA48_ECMA35, not supported */
 
                 /* The following can never be primary data syntax: */
+#ifdef WITH_SIXEL
                 DECSIXEL,
+#endif
         };
 
         DataSyntax m_primary_data_syntax{DataSyntax::ECMA48_UTF8};