Add missing semicolon

[fasterthanlime]

Hi all! We've been trying out bazel to build our Rust code, and jemallocator was the hardest to get to work, but as it turns out, this is the only code change I needed to get the whole thing working.

Hope you don't mind an extra semicolon!

[fasterthanlime]

I see https://github.com/gnzlbg/jemallocator/pull/152 (from June 9, 2020) also fixes that!