adding def to pyi-file

Author: realead

pandas/_libs/missing.pyi

@@ -14,3 +14,4 @@ def checknull(val: object, inf_as_na: bool = ...) -> bool: ...
 def isnaobj(arr: np.ndarray, inf_as_na: bool = ...) -> npt.NDArray[np.bool_]: ...
 def isnaobj2d(arr: np.ndarray, inf_as_na: bool = ...) -> npt.NDArray[np.bool_]: ...
 def is_numeric_na(values: np.ndarray) -> npt.NDArray[np.bool_]: ...
+def is_float_nan(values: np.ndarray) -> npt.NDArray[np.bool_]: ...