Add 'mount' to mkosi configuration

[utezduyar]

bootctl list is failing because this ParticleOS runtime has no usable /boot mount, so bootctl cannot enumerate the boot entries from the filesystem.

[utezduyar]

cat mkosi.local.conf 
[Distribution]
Distribution=debian

[Config]
Profiles=obs-repos-stable

[Content]
VolatilePackageDirectories=./systemd/build/mkosi.builddir/ubuntu~noble~x86-64
Packages=
        mount

[Build]
ExtraSearchPaths=./systemd/build