build: fix meson warnings
Minor touch up to resolve meson method deprecation.
Current build notice:
NOTICE: Future-deprecated features used:
* 0.56.0: {'meson.source_root'}
meson_version : '>=0.56.0'
build warning:
meson.build:41: WARNING: Project targets '>=0.56.0' but uses feature deprecated since '0.56.0': meson.source_root. use meson.project_source_root() or meson.global_source_root() instead.