audio/bespokesynth: fix build on powerpc64le

ld: error: undefined symbol: juce::pnglibNamespace::png_init_filter_functions_vsx(juce::pnglibNamespace::png_struct_def*, unsigned int)

referenced by juce_graphics.cpp

CMakeFiles/BespokeSynth.dir/usr/local/include/JUCE-7.0.2/modules/juce_graphics/juce_graphics.cpp.o:(juce::pnglibNamespace::png_read_row(juce::pnglibNamespace::png_struct_def*, unsigned char*, unsigned char*))

Using system png is no-op, since it's already installed by reverse dependencies.