package/kodi-pvr-stalker: needs libxml2

package/kodi-pvr-stalker: needs libxml2

Since
https://git.buildroot.net/buildroot/commit/package/kodi?id=bf9bfd065ba3fab328fd041ca040f2bf134fecf2
kodi itself does not depend on libxml2 anymore which previously
provided libxml2 still needed by kodi-pvr-stalker:
https://github.com/kodi-pvr/pvr.stalker/blob/Krypton/CMakeLists.txt#L12

Fixes a build error not caught by autobuilders because they do not have
host jdk installed:
CMake Error at /usr/share/cmake-3.7/Modules/FindPackageHandleStandardArgs.cmake:138 (message):
  Could NOT find LibXml2 (missing: LIBXML2_LIBRARIES LIBXML2_INCLUDE_DIR)

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>