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.
Should this have the `debRreposname := cran` line that's in several of the other genome-vendor R packages (e.g. [here](https://github.com/genome-vendor/r-cran-normt3/blob/master/debian/rules#L2) and [here](https://github.com/genome-vendor/r-bioc-cn.mops/blob/master/debian/rules#L2))?