Add librt as runtime dependency (python#19986)

`librt` is needed both at build and runtime. It's already part of
mypy-requirements.txt.

Author: cdce8p

pyproject.toml

@@ -54,6 +54,7 @@ dependencies = [
   "mypy_extensions>=1.0.0",
   "pathspec>=0.9.0",
   "tomli>=1.1.0; python_version<'3.11'",
+  "librt>=0.1.0",
 ]
 dynamic = ["version"]