fix compilation errors

Author: yamt

msgpack/_unpacker.pyx

@@ -4,7 +4,7 @@
 from cpython cimport *
 cdef extern from "Python.h":
     ctypedef struct PyObject
-    cdef int PyObject_AsReadBuffer(object o, const void* buff, Py_ssize_t* buf_len) except -1
+    cdef int PyObject_AsReadBuffer(object o, const void** buff, Py_ssize_t* buf_len) except -1
 
 from libc.stdlib cimport *
 from libc.string cimport *
@@ -95,7 +95,7 @@ def unpackb(object packed, object object_hook=None, object list_hook=None,
     cdef char* cenc = NULL
     cdef char* cerr = NULL
 
-    PyObject_AsReadBuffer(packed, <const void*>&buf, &buf_len)
+    PyObject_AsReadBuffer(packed, <const void**>&buf, &buf_len)
 
     if encoding is not None:
         if isinstance(encoding, unicode):