Services: Fix visibility of Object-derivative constructors

Derivatives of Core::Object should be constructed through
ClassName::construct(), to avoid handling ref-counted objects with
refcount zero. Fixing the visibility means that misuses like this are
more difficult.

Userland/Services/SQLServer/DatabaseConnection.h

@@ -14,6 +14,8 @@ namespace SQLServer {
 
 class DatabaseConnection final : public Core::Object {
     C_OBJECT(DatabaseConnection)
+
+public:
     ~DatabaseConnection() override = default;
 
     static RefPtr<DatabaseConnection> connection_for(int connection_id);