"async" feature deps enables for "android"

[zonyitoo]

ref shadowsocks/shadowsocks-rust#856

rust-tun/src/lib.rs

Lines 30 to 38 in b60d4c2

	#[cfg(all(
	feature = "async",
	any(
	target_os = "linux",
	target_os = "macos",
	target_os = "ios",
	target_os = "android"
	)
	))]

The async mod could be enabled for "android", but the dependencies' cfg missed target_os = "android"

[Voronar]

Packer protocol section also needs android support (maybe just add target_os = "android" together with target_os = "linux"?).

rust-tun/src/async/codec.rs

Lines 29 to 41 in b60d4c2

	// Note: the protocol in the packet information header is platform dependent.
	impl PacketProtocol {
	#[cfg(target_os = "linux")]
	fn into_pi_field(&self) -> Result<u16, io::Error> {
	match self {
	PacketProtocol::IPv4 => Ok(libc::ETH_P_IP as u16),
	PacketProtocol::IPv6 => Ok(libc::ETH_P_IPV6 as u16),
	PacketProtocol::Other(_) => Err(io::Error::new(
	io::ErrorKind::Other,
	"neither an IPv4 or IPv6 packet",
	)),
	}
	}

[zonyitoo]

@meh Hello meh, this PR is ready for review.