fix build dependencies.

There was a missing build dependency and the rules file seems to be incompatible with the other r-cran-* debian packages we have in genome-vendor. This PR fixes that.
It is not TGI-specific; it's a variable included in the [Debian packaging suggestions for R](https://wiki.debian.org/GNU_R).  That page also says that `cran` is the default, so it could be omitted if this is the correct value.