[sushi] cover-art: fix compilation warnings

[Cosimo Cecchi <cosimoc src gnome org>]

commit a5dec7e53915631cc3c182fe330ef3e0391fdbd4
Author: Cosimo Cecchi <cosimo endlessm com>
Date:   Fri Jul 20 21:34:53 2018 -0700

    cover-art: fix compilation warnings

 src/libsushi/sushi-cover-art.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/libsushi/sushi-cover-art.c b/src/libsushi/sushi-cover-art.c
index b8a35c9..0785176 100644
--- a/src/libsushi/sushi-cover-art.c
+++ b/src/libsushi/sushi-cover-art.c
@@ -470,7 +470,7 @@ cache_file_query_info_cb (GObject *source,
     file = get_gfile_for_amazon (self);
     g_error_free (error);
   } else {
-    file = g_object_ref (source);
+    file = G_FILE (g_object_ref (source));
   }
 
   try_read_from_file (self, file);