Auto merge of #924 - kristate:android_eth_p_fix, r=alexcrichton

Add ETH_* constants on Android

Adding ETH_* constants that were found here on Linux but not on Android.

Source Reference is [linux/if_ether.h](https://android.googlesource.com/platform/bionic/+/master/libc/kernel/uapi/linux/if_ether.h) on Google OpenSource.

Thank-you.

/cc nix-rust/nix#865