Ignore hylafax + iaxmodem files known to be built via ngcpcfg

As noted in https://jenkins.mgm.sipwise.com/view/Failing/job/metapackages-piuparts/25/console

| 12:29:46 10m36.7s ERROR: FAIL: debsums reports modifications inside the chroot:
| 12:29:46   /etc/init.d/ngcp-hylafaxplus
| 12:29:46   /etc/default/ngcp-iaxmodem
| 12:29:46

So let's try to ignore those files which are generated
via ngcpcfg build ... by intention and therefore the
checksum of the files gets modified.