Further reduce header dependencies

This commit removes the dependency of serialize.h on PROTOCOL_VERSION,
and makes this parameter required instead of implicit. This is much saner,
as it makes the places where changing a version number can have an
influence obvious.

src/util.h

@@ -32,7 +32,6 @@ typedef int pid_t; /* define for windows compatiblity */
 #include <openssl/ripemd.h>
 
 #include "netbase.h" // for AddTimeData
-#include "ui_interface.h"
 
 typedef long long  int64;
 typedef unsigned long long  uint64;