Must include build.dtd anyplace that uses navigator.dtd for the BuildId. Bug #26798 r=jag

suite/browser/hiddenWindow.xul

@@ -39,6 +39,8 @@ Contributor(s): ______________________________________. -->
 <!DOCTYPE window [
 <!ENTITY % brandDTD SYSTEM "chrome://global/locale/brand.dtd" >
 %brandDTD;
+<!ENTITY % buildDTD SYSTEM "chrome://global/content/build.dtd" >
+%buildDTD;
 <!ENTITY % navigatorDTD SYSTEM "chrome://navigator/locale/navigator.dtd" >
 %navigatorDTD;
 ]>