pythoncapi_compat: don't define Py_NULL if it is already defined (python#18699)

Fixes: python#18698 

This is a naive fix for the gcc 15 error when compiling for Python 3.12

Author: mr-c

mypyc/lib-rt/pythoncapi_compat.h

@@ -34,6 +34,7 @@ extern "C" {
 #  define _Py_CAST(type, expr) ((type)(expr))
 #endif
 
+#ifndef _Py_NULL
 // Static inline functions should use _Py_NULL rather than using directly NULL
 // to prevent C++ compiler warnings. On C23 and newer and on C++11 and newer,
 // _Py_NULL is defined as nullptr.
@@ -43,6 +44,7 @@ extern "C" {
 #else
 #  define _Py_NULL NULL
 #endif
+#endif
 
 // Cast argument to PyObject* type.
 #ifndef _PyObject_CAST