d/rules: Accept dots in the extended BINNMU rule

Builds in the v2019 OBS were failing in the gencontrol target since the build suffix used there is v2019.0b1 and the regest was not matching the . dot.

This was not an issue in the v2020dev0 project since its build suffix does not contain dots:

$ VERSION=4.19.37-5co1bv2020dev0b1; echo "$VERSION" | sed -rne 's,[^b]*b([a-z0-9]+)$$,\1,p'
v2020dev0b1
$ VERSION=4.19.37-5co1bv2019.0b1; echo "$VERSION" | sed -rne 's,[^b]*b([a-z0-9]+)$$,\1,p'
4.19.37-5co1b1 # ARGH
$ VERSION=4.19.37-5co1bv2019.0b1; echo "$VERSION" | sed -rne 's,[^b]*b([a-z0-9.]+)$$,\1,p'
v2019.0b1
$ VERSION=4.19.37-5co1bv2020dev0b1; echo "$VERSION" | sed -rne 's,[^b]*b([a-z0-9.]+)$$,\1,p'
v2020dev0b1