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/node/interface_ui.h

@@ -6,8 +6,6 @@
 #ifndef BITCOIN_NODE_INTERFACE_UI_H
 #define BITCOIN_NODE_INTERFACE_UI_H
 
-#include <btcsignals.h>
-
 #include <cstdint>
 #include <functional>
 #include <string>
@@ -17,6 +15,10 @@ class CBlockIndex;
 enum class SynchronizationState;
 struct bilingual_str;
 
+namespace btcsignals {
+    class connection;
+} // namespace btcsignals
+
 /** Signals for UI communication. */
 class CClientUIInterface
 {