Fix dkms source name when working from git repo

Installing the dkms package built from a git checkout fails because it doesn't use the correct module name, for example 3.9~583. Output of the dpkg-query command is for example `1:3.9~dirty-69fbb754b836710a86e83c1a9a064e54dde4c711`, the source package installs `/usr/src/libreswan-3.9~dirty` so it just needs to strip to the first hyphen.

The previous failling error message was "This package appears to be a binaries-only package" (sorry, don't more scrollback