audio/artyfx-lv2: fix build on powerpc

In file included from /wrkdirs/usr/ports/audio/artyfx-lv2/work/openAV-ArtyFX-release-1.3-24-g6010d1a/src/avtk/libs/pffft.cxx:101:
/usr/lib/clang/11.0.1/include/altivec.h:13:2: error: "AltiVec support not enabled"