package/libselinux: enable kernel selinux support for ubifs images

If BR2_TARGET_ROOTFS_UBIFS is selected, enable the following kernel options:
  - CONFIG_UBIFS_FS_XATTR
  - CONFIG_UBIFS_FS_SECURITY

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