gcc: fix _Py_atomic_compare_exchange_ptr() cast

Author: vstinner

Include/cpython/pyatomic_gcc.h

@@ -132,7 +132,7 @@ _Py_atomic_compare_exchange_ssize(Py_ssize_t *obj, Py_ssize_t *expected, Py_ssiz
 
 static inline int
 _Py_atomic_compare_exchange_ptr(void *obj, void *expected, void *desired)
-{ return __atomic_compare_exchange_n((void **)obj, expected, desired, 0,
+{ return __atomic_compare_exchange_n((void **)obj, (void **)expected, desired, 0,
                                      __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); }