multimedia/qt6-multimedia: fix build on powerpc

No AltiVec on powerpc:
In file included from /wrkdirs/usr/ports/multimedia/qt6-multimedia/work/qtmultimedia-everywhere-src-6.4.1/src/3rdparty/pffft/pffft.c:107:
/usr/lib/clang/13.0.0/include/altivec.h:13:2: error: "AltiVec support not enabled"