audio/sfizz: fix build on powerpc

CMake Error at /usr/local/share/cmake/Modules/FindOpenMP.cmake:261 (try_compile):

Failed to generate test project build system.

Call Stack (most recent call first):

/usr/local/share/cmake/Modules/FindOpenMP.cmake:537 (_OPENMP_GET_FLAGS)
cmake/SfizzDeps.cmake:7 (find_package)
CMakeLists.txt:52 (include)

Also disable VST on powerpc*, since it's disabled in plugins/CMakeLists.txt.