build: consistent debhelper 10 support

Type: fix

debian/compat contents were set to 10, but debhelper build dependency,
which is the same thing, was set to only 9. Both are not needed,
debian/compat being deprecated, so use only the latter.
At compat level 10, dh-systemd is integrated directly into debhelper, so
build dependency not needed (And in Debian Bullseye dh-systemd
transitional package is completely removed).
See: https://manpages.debian.org/testing/debhelper/debhelper.7.en.html#COMPATIBILITY_LEVELS

Signed-off-by: Nick Brown <nickbroon@gmail.com>
Change-Id: I5809b739334076d7fa8389b076a97f1648726e6e