debs: Fix build-dependencies

bd356cb seems to have introduced a typo, presumably due to a rebasing
problem in the list of build dependencies in the Debian control file.
This commit fixes it.

This patch emphasises the need for automated tests for our Debian/Ubuntu
packaging. much like we already have for Fedora.
Merge pull request #234 from wmanley/fix-deb-build-deps

debs: Fix build-dependencies