[gtk] meson.build: Fix Visual Studio builds

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

commit 2f64651a3f10d350a80e0c74c4655925e7cfb3cf
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Tue Mar 31 12:00:26 2020 +0800

    meson.build: Fix Visual Studio builds
    
    We need to make the C++ language available before we can access the C++
    compiler object...

 meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/meson.build b/meson.build
index 44274a442c..25db8e5145 100644
--- a/meson.build
+++ b/meson.build
@@ -230,6 +230,7 @@ if cc.get_id() == 'msvc'
   ]
   add_project_arguments(cc.get_supported_arguments(test_cflags), language: 'c')
 
+  add_languages('cpp')
   cxx = meson.get_compiler('cpp')
   if cxx.get_id() == 'msvc'
     add_project_arguments(cxx.get_supported_arguments(test_cflags), language: 'cpp')