Set --no-parallel, as in v10 debhelper defaults to parallel building.
This is apparently not supported by simtrace2's Makefile, it leads to
multiple non-trivial build errors. In contrib/jenkins.sh we also don't
build multiple firmwares in parallel.
Related: OS#5958
Related: https://manpages.debian.org/testing/debhelper/debhelper-compat-upgrade-checklist.7.en.html
Change-Id: I49fcc4fe9d3e795f8c3514d35ff3e2beca2917d1
Oliver Smith