gh-149044: Fix PySlot_END macro for C++

Initialize all fields in the macro. Otherwise, g++ -Wall -Wextra
complains that some fields are not initialized
[-Werror=missing-field-initializers]:

  error: missing initializer for member ‘PySlot::sl_flags’
  error: missing initializer for member ‘PySlot::<anonymous>’
  error: missing initializer for member ‘PySlot::<anonymous>’

Author: vstinner

Include/slots.h

@@ -42,7 +42,7 @@ struct PySlot {
 #define PySlot_STATIC_DATA(NAME, VALUE) \
     {.sl_id=(NAME), .sl_flags=PySlot_STATIC, .sl_ptr=(VALUE)}
 
-#define PySlot_END {0}
+#define PySlot_END {0, 0, {0}, {0}}
 
 
 // Macros without designated initializers (for C++11 and below):