Undeclared identifier `directSolver`

A user ran into this issue in Smith when they disabled Strumpack.

```
smith/ContinuationSolvers/utilities.cpp:299:13: error: use of undeclared identifier 'directSolver'
  299 |    solver = directSolver;
      |             ^
1 error generated.
 ```

On this line:

https://github.com/LLNL/ContinuationSolvers/blob/14048e7c714236127666712a07d32d5eb8b0f435/utilities.cpp#L299