libvpx.recipe: Also build a shared lib on macOS

We want both dynamic and static libraries on macOS, and only static libs on iOS, Android, Windows.

We used to have a static library on all platforms because libvpx's custom build system was too finicky regarding shared builds. With the port to Meson, for some reason the libvpx recipe was specifying only dylibs on macOS, which is not intended.

We continue to require static libs on Windows because libvpx only supports those.