Enable frame assignability.

Author: evoskuil

include/bitcoin/protocol/zmq/frame.hpp

@@ -72,7 +72,7 @@ class BCP_API frame
     bool destroy() NOEXCEPT;
 
     bool more_;
-    const bool valid_;
+    bool valid_;
 
     // Mutability is a hack around lack of const overloads in zeromq, allowing
     // the payload method to be properly const while other access is not.