flatpak-images/platform-arch-libs.bst: Add mount point for VAAPI.Intel.i386
From the commit message:
The mount point of the org.freedesktop.Platform.VAAPI.Intel.i386
extension was missing, which meant that apps couldn't actually mount
it in the intended directory.
Required by https://github.com/flathub/com.valvesoftware.Steam/pull/835.