docbook.am: --path is also required for xsltproc

The XSLT processor also needs to know about the entities defined
in xserver.ent. It removes error messages.

Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>
Signed-off-by: Keith Packard <keithp@keithp.com>

docbook.am

@@ -79,6 +79,7 @@ shelf_DATA += $(docbook:.xml=.html.db) $(docbook:.xml=.fo.db)
 # Flags for the XSL Transformation processor generating xref target databases
 XSLTPROC_FLAGS =					\
 	--path "$(XORG_SGML_PATH)/X11"			\
+	--path "$(abs_top_builddir)"			\
 	--stringparam targets.filename "$@"		\
 	--stringparam collect.xref.targets "only"	\
 	--nonet --xinclude