main: fix missing printf specifier

fixes this warning:
> ../src/main.c: In function 'capture_completed':
> ../src/main.c:321:9: error: format not a string literal and no format arguments -Werror=format-security
>   321 |         snprintf(last_path, sizeof(last_path), args->fname);
>       |         ^~~~~~~~

otherwise, `args->fname` itself would be interpreted as the specifier.

src/main.c

@@ -318,7 +318,7 @@ struct capture_completed_args {
 static bool
 capture_completed(struct capture_completed_args *args)
 {
-        snprintf(last_path, sizeof(last_path), args->fname);
+        snprintf(last_path, sizeof(last_path), "%s", args->fname);
 
         gtk_image_set_from_paintable(GTK_IMAGE(thumb_last),
                                      GDK_PAINTABLE(args->thumb));