CLN: fix build warning in c_timestamp.pyx
```
pandas/_libs/tslibs/c_timestamp.c: In function ‘__pyx_f_6pandas_5_libs_6tslibs_11c_timestamp_10_Timestamp__get_start_end_field’:
pandas/_libs/tslibs/c_timestamp.c:7257:25: warning: ‘__pyx_t_10’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 7257 |   __pyx_t_5numpy_int8_t __pyx_t_10[1];
      |  
```

Retrying fix dropped from https://github.com/pandas-dev/pandas/pull/27371

a review [comment](https://github.com/pandas-dev/pandas/pull/27371#discussion_r303179721) there disliked reintroducing `ndarray` here because the file had been converted to using memoryviews. `get_start_end_field` returns a view of type `npy_bool` so`out` should therefore be either a memoryview or an ndarray, and bool memoryviews will only be supported in cython 0.30
(see cython/cython/pull/2676).
