signals: re-add forward-declares to interface headers

The real includes were only needed temporarily while supporting btcsignals as
an alias for boost::signals2.

src/interfaces/handler.h

@@ -5,11 +5,13 @@
 #ifndef BITCOIN_INTERFACES_HANDLER_H
 #define BITCOIN_INTERFACES_HANDLER_H
 
-#include <btcsignals.h>
-
 #include <functional>
 #include <memory>
 
+namespace btcsignals {
+    class connection;
+} // namespace btcsignals
+
 namespace interfaces {
 
 //! Generic interface for managing an event handler or callback function