[calls] meson: Don't use deprecated meson.build_root()

[Evangelos Ribeiro Tzaras <devrtz src gnome org>]

commit de834bcfe990442e8a76263df4e09ac2b7671b81
Author: Evangelos Ribeiro Tzaras <devrtz fortysixandtwo eu>
Date:   Wed Dec 29 11:07:14 2021 +0100

    meson: Don't use deprecated meson.build_root()
    
    This function is deprecated since 0.56.0:
    
    WARNING: Project targeting '>= 0.56.0' but tried to use feature deprecated since
    '0.56.0': meson.build_root. use meson.project_build_root() or
    meson.global_build_root() instead.

 doc/meson.build | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/doc/meson.build b/doc/meson.build
index 31c79ae1..85bddaf6 100644
--- a/doc/meson.build
+++ b/doc/meson.build
@@ -26,9 +26,9 @@ gnome.gtkdoc('calls',
            main_xml: 'calls-docs.xml',
             src_dir: [
                        join_paths(meson.source_root(), 'src'),
-                       join_paths(meson.build_root(), 'src'),
-                       join_paths(meson.build_root(), 'src/dbus'),
-                       join_paths(meson.build_root(), 'plugins/ofono/libgdbofono'),
+                       join_paths(meson.project_build_root(), 'src'),
+                       join_paths(meson.project_build_root(), 'src/dbus'),
+                       join_paths(meson.project_build_root(), 'plugins/ofono/libgdbofono'),
                      ],
        dependencies: calls_doc_deps,
           scan_args: [