squashfs: always enable xattr support

In 785fef10 xattr support was disabled, probably because the old version
did require the xattr package to be present to support it.
In a409d76e xattr support was made optional via (lib)attr, however the
squashfs package does in fact include all of the required bits to
support xattr without the need for the attr package.

So enable xattr support by default for both the host and target
variants.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>