package/libxslt: fix build with latest libxml2

package/libxslt: fix build with latest libxml2

Build is broken since bump of libxml2 to version 2.9.11 in commit
a241dcec4188dbf30fbc8b65d7e6f2ece9da3d04 because libxslt calls the
following command "${XML_CONFIG} --libs print" which will return an
error code since
https://github.com/GNOME/libxml2/commit/2a357ab99e6f5c9196384b11cd91dd993f93014c

Fixes:
 - http://autobuild.buildroot.org/results/47ceb8c24c9ead8a450b7fea3266f760d6b77b4f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>