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/qt/bitcoin.cpp

@@ -8,6 +8,7 @@
 #include "guiutil.h"
 
 #include "init.h"
+#include "ui_interface.h"
 #include "qtipcserver.h"
 
 #include <QApplication>