Fix missing Q_DECLARE_METATYPE include in Posted board widgets

Commit 724db9ae6 ("Fix duplicated MOC declaration") consolidated
Q_DECLARE_METATYPE(RsPostedPost) into PostedItem.h and removed the
in-file declarations that PostedListWidgetWithModel.cpp and
PostedPostsModel.cpp previously carried. But both translation units
use QVariant with RsPostedPost (value<>() / fromValue()) and neither
includes PostedItem.h, directly or transitively, so the build broke
with:

  error: static assertion failed: Type is not registered, please use
  the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object
  system

Add the missing #include "PostedItem.h" to both files so the metatype
declaration is visible in those TUs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

retroshare-gui/src/gui/Posted/PostedListWidgetWithModel.cpp

@@ -53,6 +53,7 @@
 #include "PostedListWidgetWithModel.h"
 #include "PostedPostsModel.h"
 #include "BoardPostDisplayWidget.h"
+#include "PostedItem.h" // for Q_DECLARE_METATYPE(RsPostedPost)
 
 #include <algorithm>
 

retroshare-gui/src/gui/Posted/PostedPostsModel.cpp

@@ -32,6 +32,7 @@
 #include "util/DateTime.h"
 
 #include "PostedPostsModel.h"
+#include "PostedItem.h" // for Q_DECLARE_METATYPE(RsPostedPost)
 
 //#define DEBUG_CHANNEL_MODEL