haiku: Correctly set keyboard focus (thanks, Kai!).

The message sent upon the window being activated or deactivated, to trigger
the call to SDL_SetKeyboardFocus was missing a mandatory parameter. So
keyboard focus was never properly set.

Fixes Bugzilla #3658.

src/video/haiku/SDL_BWin.h

@@ -250,6 +250,7 @@ class SDL_BWin:public BDirectWindow
 
     virtual void WindowActivated(bool active) {
         BMessage msg(BAPP_KEYBOARD_FOCUS);  /* Mouse focus sold separately */
+        msg.AddBool("focusGained", active);
         _PostWindowEvent(msg);
     }