units: restore ConditionNeesUpdate=/etc in ldconfig.service (#3311)

In order to support stateless systems that support offline /usr updates
properly, let's restore the ConditionNeesUpdate=/etc line that makes sure we
are run when /usr is updated and this update needs to be propagated to the
/etc/ld.so.conf file stored in /etc.

This reverts part of #2859, which snuck this change in, but really shouldn't
have.

units/ldconfig.service

@@ -12,6 +12,7 @@ DefaultDependencies=no
 Conflicts=shutdown.target
 After=local-fs.target
 Before=sysinit.target shutdown.target systemd-update-done.service
+ConditionNeedsUpdate=|/etc
 ConditionFileNotEmpty=|!/etc/ld.so.cache
 
 [Service]