ebtables: install ethertypes

It's handy and closes bug #6608.
Thanks to Cassiano Martin <cassiano@idbinfo.com.br> for pointing it out.

[Peter: use INSTALL]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>