Add necessary headers to back-port new buffer protocol to Python 2.6
diff --git a/Include/pyerrors.h b/Include/pyerrors.h
index 73a0e36..9c2bc67 100644
--- a/Include/pyerrors.h
+++ b/Include/pyerrors.h
@@ -160,6 +160,8 @@
 PyAPI_DATA(PyObject *) PyExc_VMSError;
 #endif
 
+PyAPI_DATA(PyObject *) PyExc_BufferError;
+
 PyAPI_DATA(PyObject *) PyExc_MemoryErrorInst;
 PyAPI_DATA(PyObject *) PyExc_RecursionErrorInst;