Adding <preface> @xml:id rule to schematron integrity checks

ws/Xmlmind/Patch/040_hdmBusinessIntegrityRules.patch

@@ -28,7 +28,7 @@ diff -Naur xmlmind.orig/addon/config/docbook5/rng/V5.0/docbook.sch xmlmind/addon
 +   </s:pattern>
 +
 +   <s:pattern name="Mandatory Id definition constraint">
-+      <s:rule context="db:chapter|db:section|db:figure|db:equation|db:table|db:qandaset">
++      <s:rule context="db:preface|db:chapter|db:section|db:figure|db:equation|db:table|db:qandaset">
 +         <s:assert test="@xml:id">Each chapter,section, subection,... must have a unique id.</s:assert>
 +      </s:rule>
 +   </s:pattern>