gh-116869: Fix redefinition of the _PyOptimizerObject type (#116963)

Defining a type twice is a C11 feature and so makes the C API
incompatible with C99. Fix the issue by only defining the type once.

Example of warning (treated as an error):

    In file included from Include/Python.h:122:
    Include/cpython/optimizer.h:77:3: error: redefinition of typedef
    '_PyOptimizerObject' is a C11 feature [-Werror,-Wtypedef-redefinition]
    } _PyOptimizerObject;
    ^
    build/Include/cpython/optimizer.h:60:35: note: previous definition is here
    typedef struct _PyOptimizerObject _PyOptimizerObject;
                                    ^