[gnome-software] trivial: Fix a snap -Wsigned warning

[Richard Hughes <rhughes src gnome org>]

commit 6a3faf4f8ea08b507dd813c342fee68ac4c66b30
Author: Richard Hughes <richard hughsie com>
Date:   Mon Jun 26 20:03:46 2017 +0100

    trivial: Fix a snap -Wsigned warning

 plugins/snap/gs-plugin-snap.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/plugins/snap/gs-plugin-snap.c b/plugins/snap/gs-plugin-snap.c
index c5fa846..0095998 100644
--- a/plugins/snap/gs-plugin-snap.c
+++ b/plugins/snap/gs-plugin-snap.c
@@ -126,7 +126,7 @@ gs_plugin_snap_set_app_pixbuf_from_data (GsApp *app, const gchar *buf, gsize cou
        g_autoptr(GError) error_local = NULL;
 
        loader = gdk_pixbuf_loader_new ();
-       if (!gdk_pixbuf_loader_write (loader, buf, count, &error_local)) {
+       if (!gdk_pixbuf_loader_write (loader, (const guchar *) buf, count, &error_local)) {
                g_debug ("icon_data[%" G_GSIZE_FORMAT "]=%s", count, buf);
                g_set_error (error,
                             GS_PLUGIN_ERROR,