[fix] simple template: preferences - add missing icon_smal import

Related: https://github.com/searxng/searxng/commit/2149e88bdd64#r119535272
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/templates/simple/preferences.html

@@ -1,4 +1,4 @@
-{%- from 'simple/icons.html' import icon_big -%}
+{%- from 'simple/icons.html' import icon_small, icon_big -%}
 {%- extends "simple/page_with_header.html" -%}
 
 {%- macro tabs_open() -%}