[gdk-pixbuf/macos-ci] Try to fix libffi build

[Matthias Clasen <matthiasc src gnome org>]

commit edcb921d976eb3dddb3ce54fcc9579b53def1889
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Jan 3 15:55:48 2021 -0500

    Try to fix libffi build

 meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index 7c44860a5..ecb745186 100644
--- a/meson.build
+++ b/meson.build
@@ -4,7 +4,7 @@ project('gdk-pixbuf', 'c',
         default_options: [
           'buildtype=debugoptimized',
           'warning_level=1',
-          'c_std=c99',
+          'c_std=gnu99',
         ],
         meson_version: '>= 0.55.3')