[gtksourceview] build: use gtk4-update-icon-cache

[Christian Hergert <chergert src gnome org>]

commit bc22c231bfc7950428dade5f543b1a6453ff3e54
Author: Christian Hergert <chergert redhat com>
Date:   Mon Mar 8 19:34:05 2021 -0800

    build: use gtk4-update-icon-cache

 meson_postinstall.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson_postinstall.py b/meson_postinstall.py
index 1aec5b12..bf0d44f0 100755
--- a/meson_postinstall.py
+++ b/meson_postinstall.py
@@ -9,5 +9,5 @@ datadir = os.path.join(prefix, 'share')
 # Packaging tools define DESTDIR and this isn't needed for them
 if 'DESTDIR' not in os.environ:
     print('Updating icon cache...')
-    subprocess.call(['gtk-update-icon-cache', '-qtf',
+    subprocess.call(['gtk4-update-icon-cache', '-qtf',
                      os.path.join(datadir, 'icons', 'hicolor')])