base-files: default_postinst() force clear luci-indexcache

Inside every LuCI package you need to clear luci-indexcache and
sometimes when installing non LuCI pacakges it's also needed to clear
it. Easier put it into default_postinst().

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>

SVN-Revision: 42923