doc: Add id to index in XSLT build

That way, the HTML file name of the index will be the same as currently
for the DSSSL build.

doc/src/sgml/postgres.sgml

@@ -270,6 +270,6 @@
 
  &biblio;
  <![%include-index;[&bookindex;]]>
- <![%include-xslt-index;[<index></index>]]>
+ <![%include-xslt-index;[<index id="bookindex"></index>]]>
 
 </book>