Use same define as CPython for 'Python.h'.

Author: fangerer

graalpython/com.oracle.graal.python.cext/include/Python.h

@@ -36,8 +36,8 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  * SOFTWARE.
  */
-#ifndef PYTHON_H
-#define PYTHON_H
+#ifndef Py_PYTHON_H
+#define Py_PYTHON_H
 
 #define HAVE_UTIME_H
 #define HAVE_UNISTD_H