package/libselinux: enable kernel selinux support for squashfs images

If BR2_TARGET_ROOTFS_SQUASHFS is selected, enable the following kernel options:
  - CONFIG_SQUASHFS_XATTR

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>