[libadwaita] build: Fix cross-compilation with -Dgtk_doc=true

[Alexander Mikhaylenko <alexm src gnome org>]

commit ac080d575a058743c53f6def97ec30c97bd35e1b
Author: Michal Vasilek <michal vasilek cz>
Date:   Tue Jun 1 17:37:28 2021 +0200

    build: Fix cross-compilation with -Dgtk_doc=true
    
    We need gi-docgen on the build machine system rather than host system,
    it's a build-only dep.
    
    See https://mesonbuild.com/Reference-manual.html#dependency

 doc/meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/doc/meson.build b/doc/meson.build
index 63368183..29cc7ebe 100644
--- a/doc/meson.build
+++ b/doc/meson.build
@@ -17,6 +17,7 @@ libadwaita_toml = configure_file(
 
 dependency('gi-docgen', version: '>= 2021.1',
            fallback: ['gi-docgen', 'dummy_dep'],
+           native: true,
            required: get_option('gtk_doc'))
 
 gidocgen = find_program('gi-docgen')