[gtk+] gdk: Convince gtk-doc to include backend docs

[Matthias Clasen <matthiasc src gnome org>]

commit ea6da7b1b82d1c58b9a50fa8bdfe778efa680378
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Jan 4 15:55:09 2018 -0500

    gdk: Convince gtk-doc to include backend docs
    
    This is almost as unpleasant with meson as it is with autotools.

 docs/reference/gdk/meson.build |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gdk/meson.build b/docs/reference/gdk/meson.build
index f9150a9..3f462f2 100644
--- a/docs/reference/gdk/meson.build
+++ b/docs/reference/gdk/meson.build
@@ -87,6 +87,8 @@ gnome.gtkdoc('gdk4',
              main_xml: 'gdk4-docs.xml',
              src_dir: [
               join_paths(meson.source_root(), 'gdk'),
+              join_paths(meson.source_root(), 'gdk', 'x11'),
+              join_paths(meson.source_root(), 'gdk', 'wayland'),
               join_paths(meson.build_root(), 'gdk'),
             ],
              dependencies: libgtk_dep,