multimedia/qt6-multimedia: Adapt to local pipewire symbols

pw_init and pw_deinit were changed to pipewire_init and pipewire_deinit,
respectively, in mulimedia/pipewire so as not to conflict with symbols
from libutil.

PR: 284719
MFH: 2025Q1
(cherry picked from commit 7697c377cad948a7f9a479c29b426bbd711b851e)