meson: fix build with vpx 1.3.x
meson: fix build with vpx 1.3.x

vpx >= 1.4.0 is optional

GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good