package/libvirt: needs xsltproc

libvirt needs xsltproc since its addition in commit
ccfc90e1:

../output-1/build/libvirt-7.4.0/meson.build:807:2: ERROR: Program 'xsltproc' not found

Fixes:
 - http://autobuild.buildroot.org/results/0341be10c4c36f37b5b6486428e55170a42b7692



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>