Documentation/Makefile: add missing xsl dependencies for manpages

These stylesheets very rarely change, but when they do, it really helps if the manpages depend on them. We're casting the net a bit too wide here, since we'll only ever use a subset of the stylesheets, but since these files change so rarely, that should be ok. It's better than missing a dependency. Observe that manpage-base-url.xsl is a generated file, so we need to list it explicitly, besides the `wildcard` expression we're adding here. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>