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/node/interfaces.cpp

@@ -5,6 +5,7 @@
 #include <addrdb.h>
 #include <banman.h>
 #include <blockfilter.h>
+#include <btcsignals.h>
 #include <chain.h>
 #include <chainparams.h>
 #include <common/args.h>
@@ -69,8 +70,6 @@
 #include <optional>
 #include <utility>
 
-#include <boost/signals2/signal.hpp>
-
 using interfaces::BlockRef;
 using interfaces::BlockTemplate;
 using interfaces::BlockTip;