Add new HEIC magic numbers for 10 bit images

[der-eismann]

Hey everyone, we know that this project is in a maintenance state, but we lack the resources to migrate to the recommended vipsgen library and we need to make govips work with 10 bit HEIC images from Apple devices, which have a different magic number, see here: https://github.com/strukturag/libheif/issues/83#issuecomment-421427091

Since they are supported by libheif it should be a no-brainer to add them: https://github.com/strukturag/libheif/blob/master/go/heif/heif.go#L1508-L1517

It was tested internally with a 10 bit image successfully.