fix(prqlc-python): add missing type annotation for pl_to_prql (#4588)

prqlc/bindings/prqlc-python/python/prqlc/__init__.pyi

@@ -12,6 +12,7 @@ class CompileOptions:
 def compile(prql_query: str, options: Optional[CompileOptions] = None) -> str: ...
 def prql_to_pl(prql_query: str) -> str: ...
 def pl_to_rq(pl_json: str) -> str: ...
+def pl_to_prql(pl_json: str) -> str: ...
 def rq_to_sql(rq_json: str) -> str: ...
 def get_targets() -> List[str]: ...