Fix build breakage with Python 2.4.

Fix build breakage with Python 2.4.

With Python 2.4, we see this build failure:

./python/python-internal.h: In function 'gdb_Py_DECREF':
./python/python-internal.h:179: warning: dereferencing 'void *' pointer
./python/python-internal.h:179: error: request for member 'ob_refcnt' in something not a structure or union

Python 2.4 forgets to cast 'op' to PyObject pointer on the ob_refcnt
accesses:

  #define Py_DECREF(op)                                   \
          if (_Py_DEC_REFTOTAL  _Py_REF_DEBUG_COMMA       \
              --(op)->ob_refcnt != 0)                     \
                  _Py_CHECK_REFCNT(op)                    \
          else                                            \
                  _Py_Dealloc((PyObject *)(op))

...

  #define _Py_CHECK_REFCNT(OP)                                    \
  {       if ((OP)->ob_refcnt < 0)                                \
                  _Py_NegativeRefcount(__FILE__, __LINE__,        \
                                       (PyObject *)(OP));         \
  }

Python 2.7:

  #define Py_DECREF(op)                                   \
      do {                                                \
          if (_Py_DEC_REFTOTAL  _Py_REF_DEBUG_COMMA       \
          --((PyObject*)(op))->ob_refcnt != 0)            \
              _Py_CHECK_REFCNT(op)                        \
          else                                            \
          _Py_Dealloc((PyObject *)(op));                  \
      } while (0)

...

  #define _Py_CHECK_REFCNT(OP)                                    \
  {       if (((PyObject*)OP)->ob_refcnt < 0)                             \
                  _Py_NegativeRefcount(__FILE__, __LINE__,        \
                                       (PyObject *)(OP));         \
  }

gdb/
2013-05-30  Pedro Alves  <palves@redhat.com>

	* python/python-internal.h (gdb_Py_DECREF): Cast OP to PyObject
	pointer.