patch 9.0.1709: dynamic build with python 3.12 breaks

patch 9.0.1709: dynamic build with python 3.12 breaks

Problem: dynamic build with python 3.12 breaks
Solution: if_python3.c: Fix building dynamic Python3 interpreter

There are new extern global variables defined in python3 development
files, which types are within python3 library, so they break dynamic
python3 interpret Vim plugin.

Since the variables are used in macro `Py_SIZE` which is used in other
python3 headers, the dummy variables have to defined before including
Python.h.

closes: #12660

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Zdenek Dohnal <zdohnal@redhat.com>