meson: Fix warning about build_root() and source_root()

meson: Fix warning about build_root() and source_root()

```
../meson.build:384: WARNING: Project targeting '>= 0.60.0' but tried to use feature deprecated since '0.56.0': meson.build_root. use meson.project_build_root() or meson.global_build_root() instead.
../meson.build:385: WARNING: Project targeting '>= 0.60.0' but tried to use feature deprecated since '0.56.0': meson.source_root. use meson.project_source_root() or meson.global_source_root() instead.
...
WARNING: Deprecated features used:
 * 0.56.0: {'meson.source_root', 'meson.build_root'}
 ```

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1977>