Add missing virtual destructor [blocks: #2310]

[tautschnig]

8a37a2d made what() virtual, but failed to add a virtual destructor.

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• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 6e37d0e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90945382