discovery: disable invitation button when no user is selected

Change-Id: Ie9a1a51828ee05ae0140a0077587695b4a74b854
Refs: #2483
diff --git a/src/discovery-panel.hpp b/src/discovery-panel.hpp
index f15906b..1b5bd92 100644
--- a/src/discovery-panel.hpp
+++ b/src/discovery-panel.hpp
@@ -138,6 +138,8 @@
   QStringList m_rosterList;
   QString     m_chatroom;
   QString     m_participant;
+
+  bool m_isParticipant;
 };
 
 } // namespace chronochat