ProjectExplorer: Fix compile for older gcc

Issue was introduced in 5003bc60d0ec8188e0f4b9458b821fb7ffb55de9;

Change-Id: Ibe016f884e4eea8093173ced497ee4c62f1265c3
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/projectexplorer/projectexplorer.cpp

@@ -1144,7 +1144,7 @@ bool ProjectExplorerPlugin::initialize(const QStringList &arguments, QString *er
 
     expander->registerVariable(Constants::VAR_CURRENTPROJECT_NAME,
         tr("The current project's name."),
-        []() -> QString {
+        [this]() -> QString {
             Project *project = ProjectExplorerPlugin::currentProject();
             return project ? project->displayName() : QString();
         });