[gtk/matthiasc/for-master] Fix the build

[Matthias Clasen <matthiasc src gnome org>]

commit 32191bc18ea85201f6b1a003d4cb465634363f11
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Sep 28 08:25:00 2021 -0400

    Fix the build
    
    gcc was complaining here with -Werror=return-type.

 gtk/gtkcsstransformvalue.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gtk/gtkcsstransformvalue.c b/gtk/gtkcsstransformvalue.c
index f50873328a..6cec8cd7c0 100644
--- a/gtk/gtkcsstransformvalue.c
+++ b/gtk/gtkcsstransformvalue.c
@@ -239,6 +239,8 @@ gtk_css_transform_apply (const GtkCssTransform   *transform,
       g_assert_not_reached ();
       break;
     }
+
+  return NULL;
 }
 
 /* NB: The returned matrix may be invalid */