exclude: Omit systemd-ssh-generator

This thing is pointless.

Unfortunately, we cannot use Portage's `INSTALL_MASK` feature as it
doesn't work for symbolic links. Since _systemd_ installs symlinks in
`/etc/ssh` that point to files we would mask, those symlinks would point
to nothing, which would cause `sshd` to fail to start as it is unable to
open those files.  Thus, we have to omit these files by excluding them
from the squashfs image.

squashfs.exclude

@@ -0,0 +1,5 @@
+etc/ssh/ssh_config.d/20-systemd-ssh-proxy.conf
+etc/ssh/sshd_config.d/20-systemd-userdb.conf
+usr/lib/systemd/ssh_config.d
+usr/lib/systemd/sshd_config.d
+usr/lib/systemd/system-generators/systemd-ssh-generator