libxslt: tell ./configure where libxml is

libxslt is currently configured with --with-libxml-include-prefix,
which allows libxslt ./configure to find libxml headers. However, the
build of libxslt fails because it doesn't find the library itself.

Therefore, instead of using --with-libxml-include-prefix, we switch to
the more generic --with-libxml-prefix.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>