docs: fix Makefile dependency for user manual

We use our custom xsl file to build the user manual, so make
sure we depend on it. We don't use it anywhere else, so we
can stick it straight in the rule.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -279,7 +279,7 @@ $(patsubst %,%.html,$(API_DOCS) technical/api-index): %.html : %.txt
 XSLT = docbook.xsl
 XSLTOPTS = --xinclude --stringparam html.stylesheet docbook-xsl.css
 
-user-manual.html: user-manual.xml
+user-manual.html: user-manual.xml $(XSLT)
 	$(QUIET_XSLTPROC)$(RM) $@+ $@ && \
 	xsltproc $(XSLTOPTS) -o $@+ $(XSLT) $< && \
 	mv $@+ $@