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.
diff --git a/src/video/haiku/SDL_BWin.h b/src/video/haiku/SDL_BWin.h
index f2f8287..830ebe2 100644
--- a/src/video/haiku/SDL_BWin.h
+++ b/src/video/haiku/SDL_BWin.h
@@ -250,6 +250,7 @@
 
     virtual void WindowActivated(bool active) {
         BMessage msg(BAPP_KEYBOARD_FOCUS);  /* Mouse focus sold separately */
+        msg.AddBool("focusGained", active);
         _PostWindowEvent(msg);
     }