gh-121617: Include string.h in pwdmodule.c if _Py_TYPEOF is undefined

[fuhsnn]

If _Py_TYPEOF is undefined, Py_CLEAR falls back to an implementation with memcpy() call; conditionally include the missing string.h dependency.

• Issue: memcpy() usage without #include <string.h> in pwdmodule.c #121617

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[vstinner]

This change is outdated: the pwd extension is no longer built with the limited C API.

Also, it's not the proper fix: Python.h should include <string.h> if the _Py_TYPEOF macro is not defined. I wrote #144666 change to do that.

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