Add /usr to non-unmountable list

Debian's systemd has split-usr enabled as both Debian and in principle Apertis
support both a split and a merged-usr setup. This mostly adds search paths to
system (e.g. to look at both `/bin/` and `/usr/bin`) however it also means that
if `/usr` is a mountpoint system will try to unmount it.  Unfortunately this causes
issues at shutdown for systems with a merged-usr layout as basic libraries (e.g. libc)
are located in `/usr` making it impossible to unmount.

For Apertis we don't support `/usr` being a seperate partition, the only time
`/usr` is on a mountpoint is when using ostree images (where it is a bind mount
and which use a merged-usr layout). So also add `/usr` to the list of paths that
are considered unmountable even with split-usr support enabled in systemd.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>