sysutils/s6: add a service dependency on ldconfig.

The last skaware release changed the linkage for internal libs
from static to dynamic. Locating these in
/usr/local/lib/ requires a valid /var/run/ld-elf.so.hints.

PR: 284677
Reported by: Saj Goonatilleke <sg@redu.cx>
MFH: 2025Q1