Update buildsystem.py: fix debhelper-compat detection

debexpo doesn't report debhelper-compat version correctly when there are other packages before "debhelper-compat (= 12)".

It's a quick solution, probably good enough.

Disclaimer: I don't have a running debexo environment, so the patch hasn't been tested yet. I've tested the regexp separately taking the string from .dsc file, as debexpo does.