Since we invoke the ngcp-sercmd command only in the postinst
script and not in preinst it's ought to be enough to depend
on the ngcp-system-tools-* package, quoting Debian Policy,
section 7.2:
| Pre-Depends are also required if the preinst script depends
| on the named package. It is best to avoid this situation if possible.
As piuparts doesn't support resolving Pre-Dependencies yet
this also solves our failing piuparts runs.
Change-Id: Ife43bfb7547c3331d541723782d8b374894e30b5