Add value_or to optional.pxd

Summary: Expose the call to folly::Optional::value_or() in the Cython bindings

Reviewed By: nanshu

Differential Revision: D18526111

fbshipit-source-id: f2694ca589bf73653bc86d5c98b52e15406bed77

folly/python/optional.pxd

@@ -18,6 +18,7 @@ cdef extern from "folly/Optional.h" namespace "folly" nogil:
         cOptional(T val)
         bint has_value()
         T value()
+        T value_or(T&& altValue)
         T* get_pointer()
         void reset()
         void assign(T&& newValue)