lib: fix a few compiler warnings

WARN_C(XX)FLAGS is really strict.

lib/mu-store.cc

@@ -1,6 +1,6 @@
 /* -*-mode: c++; tab-width: 8; indent-tabs-mode: t; c-basic-offset: 8-*- */
 /*
-** Copyright (C) 2008-2013 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2008-2017 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -33,7 +33,6 @@
 #include "mu-store.h"
 #include "mu-store-priv.hh" /* _MuStore */
 
-
 #include "mu-msg.h"
 #include "mu-msg-part.h"
 #include "mu-store.h"
@@ -66,8 +65,6 @@ mu_store_unref (MuStore *store)
 }
 
 
-
-
 static char*
 xapian_get_metadata (const gchar *xpath, const gchar *key)
 {