wayland: Make KWinDisplay less chatty

There is no point for printing a warning if kwin cannot read the exe
symlink in /proc. It may happen for processes such as polkit agent.

src/wayland_server.cpp

@@ -143,7 +143,7 @@ public:
         }
 
         if (client->executablePath().isEmpty()) {
-            qCWarning(KWIN_CORE) << "Could not identify process with pid" << client->processId();
+            qCDebug(KWIN_CORE) << "Could not identify process with pid" << client->processId();
             return false;
         }