extra: generate the theme cache var with efreet api

there can be some people who are changing the cache home directory to
some other value than the standard xdg stuff.

src/lib/extra.c

@@ -240,7 +240,7 @@ _theme_cache_path_get()
    char *path;
 
    path = malloc(PATH_MAX * sizeof(char));
-   sprintf(path, "%s/.cache/%s/%s.json", eina_environment_home_get(), PACKAGE_NAME, "themes");
+   sprintf(path, "%s/%s/%s.json", efreet_cache_home_get(), PACKAGE_NAME, "themes");
 
    return path;
 }