recipes/gst-libav: Ignore iOS optimization warnings
recipes/gst-libav: Ignore iOS optimization warnings

With never clang, we now get warnings about optimizations that were
previously silently ignored.

nirbheek/cerbero,fluendo/cerbero,atsushieno/cerbero,atsushieno/cerbero,GStreamer/cerbero,centricular/cerbero,fluendo/cerbero,GStreamer/cerbero,centricular/cerbero,atsushieno/cerbero,GStreamer/cerbero,centricular/cerbero,centricular/cerbero,fluendo/cerbero,fluendo/cerbero,atsushieno/cerbero,atsushieno/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,centricular/cerbero,nirbheek/cerbero,fluendo/cerbero,nirbheek/cerbero