Add move ctor and makeEmpty method declarations to cFollyPromise

Summary: Add cFollyPromise method declarations that are needed to create an empty `folly::Promise`

Reviewed By: Gownta

Differential Revision: D30519790

fbshipit-source-id: 46092d5a400b0fccc11a9e77271a18b4f6fa3f6d

folly/python/__init__.pxd

@@ -31,10 +31,14 @@ cdef extern from "folly/Unit.h" namespace "folly":
 cdef extern from "folly/futures/Promise.h" namespace "folly":
     cdef cppclass cFollyPromise "folly::Promise"[T]:
         cFollyPromise()
+        cFollyPromise(cFollyPromise&&)
         void setValue[M](M& value)
         void setException[E](E& value)
         cFollyFuture[T] getFuture()
 
+        @staticmethod
+        cFollyPromise[T] makeEmpty()
+
 cdef extern from "folly/Executor.h" namespace "folly":
     cdef cppclass cFollyExecutor "folly::Executor":
         pass