doc: Call xmllint for validity also in the fop build

This was somehow missed in commit
5d93ce2.

Author: petere

doc/src/sgml/Makefile

@@ -271,9 +271,11 @@ htmlhelp: stylesheet-hh.xsl postgres.xml
 	$(XSLTPROC) $(XSLTPROCFLAGS) $^
 
 %-A4.fo.tmp: stylesheet-fo.xsl %.xml
+	$(XMLLINT) --noout --valid $*.xml
 	$(XSLTPROC) $(XSLTPROCFLAGS) --stringparam paper.type A4 -o $@ $^
 
 %-US.fo.tmp: stylesheet-fo.xsl %.xml
+	$(XMLLINT) --noout --valid $*.xml
 	$(XSLTPROC) $(XSLTPROCFLAGS) --stringparam paper.type USletter -o $@ $^
 
 FOP = fop