libnftl: loosen threads dependency

nftables works fine without threads, only the XML parsing support in
libnftl requires -lpthread so move the depends into
BR2_PACKAGE_LIBNFTNL_XML.

[Thomas: tweak commit title and log.]

Signed-off-by: Alexander Clouter <alex+buildroot@digriz.org.uk>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>