git: unignore src/build in case user ignores build

[jaraco]

After checking out this project, I found that files in src/build weren't being tracked and that src appeared empty in my editor. Turns out, I used a common pattern of ignoring build in my global gitignore (which I manage centrally rather than copying and mutating boilerplate across hundreds of projects).

This change ensures that if the user has ignored build, the src/build folder won't be ignored in this project.

[henryiii]

Ah, good idea.

FYI, I try to ignore such global ignores because it causes individual projects to have incomplete ignores, which hurts users who don't have the global setup. (I do have a few things, like .DSStore, but try to keep it limited).

[FFY00]

This makes sense, thanks!