x11/keymap: don't forget to add the vmod offset in get_vmods

The first 8 modifiers in keymap->mods are the real modifiers; the virtual
 modifiers are then at slots 8-24. But XkbGetMap's virtualMods mask
starts the virtual modifiers at zero, so we need to add an offset (like
we do correctly in get_vmod_names()).

https://github.com/xkbcommon/libxkbcommon/issues/9

Reported-by: @rtcm
Signed-off-by: Ran Benita <ran234@gmail.com>