signals: use forwarding header for boost signals

For now, including btcsignals.h simply includes boost's signals. A follow-up
commit will replace the implementation.

src/noui.cpp

@@ -5,15 +5,13 @@
 
 #include <noui.h>
 
+#include <btcsignals.h>
 #include <logging.h>
 #include <node/interface_ui.h>
 #include <util/translation.h>
 
 #include <string>
 
-#include <boost/signals2/connection.hpp>
-#include <boost/signals2/signal.hpp>
-
 /** Store connections so we can disconnect them when suppressing output */
 boost::signals2::connection noui_ThreadSafeMessageBoxConn;
 boost::signals2::connection noui_ThreadSafeQuestionConn;