gitignore: remove src (#325)
gitignore: remove src (#325)

Now that we have an empty src/lib.rs for each exercise as of
https://github.com/exercism/rust/pull/270, we should ease the process of
adding the file by no longer ignoring it.
exercism/xrust,petertseng/exercism-rust,sacherjj/rust,sacherjj/rust,sacherjj/rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust