package/xmlstarlet: fix static linking

The static linking with libxml2 and lib(e)xslt (--enable-static-libs)
need to be enabled when BR2_PREFER_STATIC_LIB is set.

Fixes:
http://autobuild.buildroot.net/results/0fd/0fd4a65ab98a4cf3fe893c29628608a71e96ecbc/build-end.log



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>