meson: don't error out if sidplay library is not found
meson: don't error out if sidplay library is not found

GrokImageCompression/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly,GStreamer/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GStreamer/gst-plugins-ugly,StreamUtils/gst-plugins-ugly,GStreamer/gst-plugins-ugly,GrokImageCompression/gst-plugins-ugly