fix compile error

Summary: explicitly move exception_ptr arg to fix error

Reviewed By: Orvid

Differential Revision: D22528984

fbshipit-source-id: e72e6008e6a899663970c6dff43c8790d044dd69

folly/experimental/pushmi/o/submit.h

@@ -326,7 +326,7 @@ struct get_fn {
       *ep_ = std::make_exception_ptr(e);
     }
     void operator()(std::exception_ptr ep) const noexcept {
-      *ep_ = ep;
+      *ep_ = std::move(ep);
     }
   };