Add empty IOBuf ctor to folly cython, to match IOBuf.h

Summary: Title

Reviewed By: yfeldblum

Differential Revision: D33873064

fbshipit-source-id: 65b16ad30a7bf9f2b9e23857f072d33c94098899

folly/python/iobuf.pxd

@@ -24,6 +24,7 @@ from cython.view cimport memoryview
 
 cdef extern from "folly/io/IOBuf.h" namespace "folly":
     cdef cppclass cIOBuf "folly::IOBuf":
+        cIOBuf()
         uint64_t length()
         const_uchar* data()
         bint empty()