graphics/libplacebo: unbreak version without WRKDIRPREFIX

[vo/gpu/libplacebo] Initialized libplacebo 12.3-eol-13663-gde305010502d (API v292)

Reported by: Kevin Oberman (via bug 272709)

(cherry picked from commit 6b6bfa927bdf60136060f07ae67ea175abbb31b8)