Use OPENVPN_ETH_P_* so that <netinet/if_ether.h> is unecessary

Use OPENVPN_ETH_P_* so that <netinet/if_ether.h> is unecessary

Fixes compilation error on linux with musl libc because of
conflicting ethhdr declarations in <netinet/if_ether.h> and
<linux/if_ether.h> which is included from <linux/if_tun.h>

Signed-off-by: Felix Janda <felix.janda@posteo.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20150516134604.GA2302@euler>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9690
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit ddb1f20a9ddbb94956c9f7b1115c89543d9b411a)