xmlto: convert output path to unix path on cygwin/msys
When xmlto is invoked with "-o C:/src/build", the
resulting files end up under a subdirecty "C:/src/build/" of the current
working directory.
Convert the argument to a unix path on cygwin/msys.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>