k-order MEX: mutex was not correctly locked in initialization

Since the lock was not assigned to a variable, it was a temporary value and
would thus be immediately released.

Problem detected through a warning produced by GCC 14.

mex/sources/libkorder/tl/tl_static.cc

 /*
  * Copyright © 2004 Ondra Kamenik
- * Copyright © 2019-2022 Dynare Team
+ * Copyright © 2019-2024 Dynare Team
  *
  * This file is part of Dynare.
  *
@@ -55,7 +55,7 @@ init(int dim, int nvar)
     throw TLException(__FILE__, __LINE__,
                       "Problem too large, you should decrease the approximation order");
 
-  std::lock_guard<std::mutex> {mut};
+  std::lock_guard<std::mutex> lk {mut};
   ebundle.generateUpTo(dim);
   pbundle.generateUpTo(dim);