rfb: Avoid unused argument warnings in headers

Don't force users of these headers to squash compiler
warnings about unused arguments, annotate with __unused_attr.
diff --git a/common/rfb/InputHandler.h b/common/rfb/InputHandler.h
index b5e5e87..727f69b 100644
--- a/common/rfb/InputHandler.h
+++ b/common/rfb/InputHandler.h
@@ -25,15 +25,19 @@
 
 #include <rdr/types.h>
 #include <rfb/Rect.h>
+#include <rfb/util.h>
 
 namespace rfb {
 
   class InputHandler {
   public:
     virtual ~InputHandler() {}
-    virtual void keyEvent(rdr::U32 key, bool down) {}
-    virtual void pointerEvent(const Point& pos, int buttonMask) {}
-    virtual void clientCutText(const char* str, int len) {}
+    virtual void keyEvent(rdr::U32 __unused_attr key,
+                          bool __unused_attr down) { }
+    virtual void pointerEvent(const Point& __unused_attr pos,
+                              int __unused_attr buttonMask) { }
+    virtual void clientCutText(const char* __unused_attr str,
+                               int __unused_attr len) { }
   };
 
 }