extras/package/snap: Add `layout:` to symlink `/usr/share/libdrm`

When running the VLC snap from the command line, the following error message:

/usr/share/libdrm/amdgpu.ids: No such file or directory

This is due to a missing layout: field. The program is looking in /usr/share/libdrm/, but the file is actually in $SNAP/usr/share/libdrm/, so layout: allows creating a symlink to this location.