libcheese: Add GtkWidget cast to avoid an incompatible-pointer-types error

Submitted upstream: <https://gitlab.gnome.org/GNOME/cheese/-/merge_requests/70>

diff --git a/libcheese/cheese-flash.c b/libcheese/cheese-flash.c
index 59959dbd3414eafa..e402322166614276 100644
--- a/libcheese/cheese-flash.c
+++ b/libcheese/cheese-flash.c
@@ -132,7 +132,7 @@ cheese_flash_set_property (GObject      *object,
       GObject *parent;
       parent = g_value_get_object (value);
       if (object != NULL)
-        priv->parent = g_object_ref (parent);
+        priv->parent = GTK_WIDGET (g_object_ref (parent));
       else
         priv->parent = NULL;
     }