Meson: Squash remaining namespaces

Change-Id: I44b8ba867f01dfd917a0d1c53c9487865b781d2a
Reviewed-by: hjk <hjk@qt.io>

src/plugins/mesonprojectmanager/buildoptionsmodel.h

@@ -13,8 +13,7 @@
 #include <QItemEditorFactory>
 #include <QStyledItemDelegate>
 
-namespace MesonProjectManager {
-namespace Internal {
+namespace MesonProjectManager::Internal {
 
 class CancellableOption
 {
@@ -164,5 +163,4 @@ public:
                       const QModelIndex &index) const override;
 };
 
-} // namespace Internal
-} // namespace MesonProjectManager
+} // namespace MesonProjectManager::Internal