Add virtual destructor in GtirbBuilder

[MatthiasJReisinger]

hi :) while reading through the code I just noticed that GtirbBuilder is currently missing a virtual destructor which could lead to undefined behaviour if a sub-class would be deleted polymorphically. Even though the current code does not seem to contain any polymorphic deletions of GtirbBuilder it probably doesn't hurt to have the virtual destructor to avoid potential trouble :)

[eschulte]

Hi Matthias,

Thanks for the contribution! Unfortunately, and I know this is a little bit ridiculous for a 1-line patch, we have a CLA that we ask every contributor sign and email to CLA@GrammaTech.com (see our CONTRIBUTING.md). I know this is a lot compared to the size of the patch, but once you've signed it we can easily accept future larger patches 😀.

[MatthiasJReisinger]

ah ok thx, I'll have a look at it then :sweat_smile:

[aeflores]

This has been addressed