[gtk] demos/constraint-editor: Fix linking on MSVC builds

[Chun-wei Fan <fanchunwei src gnome org>]

commit a20fd5e14e0b5e41c1b9eb15aa348ba1076e15fb
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Mon Jul 8 18:37:27 2019 +0800

    demos/constraint-editor: Fix linking on MSVC builds
    
    We must apply the /entry:mainCRTStartup linker flag in order to link GTK
    apps in pure GUI form.  This follows what is done in demos/gtk-demo,
    etc.

 demos/constraint-editor/meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/demos/constraint-editor/meson.build b/demos/constraint-editor/meson.build
index af66846437..ce51325b37 100644
--- a/demos/constraint-editor/meson.build
+++ b/demos/constraint-editor/meson.build
@@ -16,4 +16,5 @@ executable('gtk4-constraint-editor',
            dependencies: libgtk_dep,
            include_directories: confinc,
            gui_app: true,
+           link_args: extra_demo_ldflags,
            install: false)