include mdb.h because soon the interface file will not be including it

mailnews/db/msgdb/public/nsMsgThread.h

@@ -22,6 +22,7 @@
 #include "nsIMsgThread.h"
 #include "nsString.h"
 #include "MailNewsTypes.h"
+#include "mdb.h"
 
 class nsIMdbTable;
 class nsIMessage;