build-sys: Ensure systemd-sysv-install symlink does not have //

See: ostreedev/ostree-rs-ext#182

In https://github.com/ostreedev/ostree we generate a cryptographic
checksum over files and symlinks, and directories.

ostree does not currently perform any canonicalization on symlinks;
we'll respect and honor whatever bytes we're provided as input,
and replicate that on the target.

We're using the Rust tar crate to do tar serialization,
which has so far worked fine...except, I hit this corner case:

```
[root@cosa-devsh ~]# rpm -qf /usr/lib/systemd/systemd-sysv-install
chkconfig-1.13-2.el8.x86_64
[root@cosa-devsh ~]# ll /usr/lib/systemd/systemd-sysv-install
lrwxrwxrwx. 2 root root 24 Nov 29 18:08 /usr/lib/systemd/systemd-sysv-install -> ../../..//sbin/chkconfig
[root@cosa-devsh ~]#
```

But, using `set_link_name` to write the tarball, we end up with
the canonicalized path `../../../sbin/chkconfig` - i.e. without the
double `//`.  This breaks the checksum.

Now, I am a bit tempted to change ostree to do canonicalization.  But
even if we did, I'd need to *exactly* match what tar-rs is doing.

For the record, this is *the only* non-canonical symlink in Fedora
and RHEL CoreOS today.

Makefile

@@ -60,7 +60,7 @@ install:
 	[ -d $(DESTDIR)/$(SYSTEMDUTILDIR) ] || mkdir -p -m 755 $(DESTDIR)/$(SYSTEMDUTILDIR)
 
 	install -m 755 $(PROG) $(DESTDIR)/$(SBINDIR)/$(PROG)
-	ln -s ../../../$(SBINDIR)/$(PROG) $(DESTDIR)/$(SYSTEMDUTILDIR)/systemd-sysv-install
+	ln -s ../../..$(SBINDIR)/$(PROG) $(DESTDIR)/$(SYSTEMDUTILDIR)/systemd-sysv-install
 
 	install -m 755 ntsysv $(DESTDIR)/$(SBINDIR)/ntsysv
 	install -m 755 alternatives $(DESTDIR)/$(SBINDIR)/alternatives