package/rpm: use external xz library

Internal xz backend is statically linked and need a static
libintl library when BR2_NEEDS_GETTEXT_IF_LOCALE is set.
But for a shared only build, there is no static libintl available
in the sysroot.

Use the external xz library provided by Buildroot instead of
using the internal xz backend.

Fixes:
http://autobuild.buildroot.net/results/2c6/2c61562008418ff58fa3085a3342b0e304923073/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>