Fix md5sums generation for deb packages

Since version 1.17.2, dpkg includes a `--verify` operation mode which is roughly comparable to what `rpm --verify` does. Among the checks, it verifies MD5 sums of installed files using `/var/lib/dpkg/info/<package>.md5sums` like debsums used to do.

Unfortunately, `dpkg --verify` *insists* on having the md5sums control file formatted exactly as it would be produced by a run of md5sum (hash, two spaces, file path), if a single md5sums file is badly formatted it will die with a "missing value separator" error.

This PR addresses this (debatable) behavior by adding the missing space. It should be noted that this is probably a dpkg issue (md5sum, debsums and lintian itself are more forgiving), but the aforementioned version is already shipping on recent Ubuntu versions and it would be troublesome to get it fixed.