fix maildir driver's debug flag

copy-pasto; it certainly wasn't meant to respond to --debug-sync.
the problem was barely noticeable, as the maildir driver's only debugs
are in the rarely triggered rescan path, apart from the flags usually
being used en bloc anyway.

amends 0e1f8f9a.

src/drv_maildir.c

@@ -113,7 +113,7 @@ debug( const char *msg, ... )
 	va_list va;
 
 	va_start( va, msg );
-	vdebug( DEBUG_SYNC, msg, va );
+	vdebug( DEBUG_MAILDIR, msg, va );
 	va_end( va );
 }