[Minor] Debian: set fixfilepath reproducible build option

[Minor] Debian: set fixfilepath reproducible build option

* Enable -ffile-prefix-map to ensure that assert statements in
  contrib/libev/ev.c and other places do not store the full file path.
  This ensures reproducible builds, invariant of the build directory.
  Supported since dpkg 1.19.1, including Debian buster and Ubuntu 19.04.
* Enable all hardening features while at it, this adds bindnow only. See
  https://manpages.debian.org/buster/dpkg-dev/dpkg-buildflags.1.en.html