fix for Qt 5.11

Quote from Arch mailing list:
- there's been a huge header cleanup in Qt modules. Expect build failures for applications that rely on transitive includes instead of declaring all required headers. Those need to be fixed upstream by explicitely adding the missing includes.

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

@@ -22,6 +22,7 @@
  */
 
 #include <QDateTime>
+#include <QStyle>
 
 #include "rshare.h"
 #include "PostedItem.h"

retroshare-gui/src/gui/chat/ChatTabWidget.cpp

@@ -20,6 +20,8 @@
  *  Boston, MA  02110-1301, USA.
  ****************************************************************/
 
+#include <QTabBar>
+
 #include "ChatTabWidget.h"
 #include "ui_ChatTabWidget.h"
 #include "ChatDialog.h"

retroshare-gui/src/gui/feeds/GxsChannelPostItem.cpp

@@ -23,6 +23,7 @@
 
 #include <QTimer>
 #include <QFileInfo>
+#include <QStyle>
 
 #include "rshare.h"
 #include "GxsChannelPostItem.h"

retroshare-gui/src/gui/feeds/GxsForumMsgItem.cpp

@@ -23,6 +23,7 @@
 
 #include <QTimer>
 #include <QFileInfo>
+#include <QStyle>
 
 #include "rshare.h"
 #include "GxsForumMsgItem.h"