Add pref to REQUIRES to fix senna bustage

docshell/shistory/src/Makefile.in

@@ -29,7 +29,7 @@ include $(DEPTH)/config/autoconf.mk
 MODULE		= shistory
 LIBRARY_NAME	= shistory
 IS_COMPONENT	= 1
-REQUIRES	= xpcom necko dom js layout docshell locale widget uriloader
+REQUIRES	= xpcom necko dom js layout docshell locale widget uriloader pref
 
 CPPSRCS		= nsSHEntry.cpp        \
             nsSHTransaction.cpp   \