Use the correct file descriptor

Index: libs/maildrop/maildir.C
--- libs/maildrop/maildir.C.orig
+++ libs/maildrop/maildir.C
@@ -308,7 +308,7 @@ void	Maildir::MaildirSave()
 			dir.resize(q-p);
 
 #if EXPLICITDIRSYNC
-			int syncfd=open(dir, O_RDONLY);
+			int syncfd=open(dir.c_str(), O_RDONLY);
 
 			if (syncfd >= 0)
 			{