[4.15-stable 1/1] uapi/if_ether.h: move __UAPI_DEF_ETHHDR libc define

[Mark Asselstine]

Please promote commit da360299b6734135a5f66d7db458dcc7801c826a [uapi/
if_ether.h: move __UAPI_DEF_ETHHDR libc define] to linux-4.15.y stable. 
Without this fix users of the uapi headers will run into the issue as 
described in the commit log. Specifically we have seen this with building 
ebtables.

Thanks,
Mark Asselstine