gypsy: add dependency on libxslt-native

* it's incorrectly used in build (and just "o" is used instead of xslt
  command):
  | o gypsy-discovery.xml ../docs/tools/spec-strip-docs.xsl gypsy-discovery-full.xml
  | /bin/bash: o: command not found

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>