Add SO_REUSEPORT for Android

The `net2` crate currently fails to cross-compile to Android with
``error: unresolved name `libc::SO_REUSEPORT` ``

According to http://stackoverflow.com/a/14388707/1162888,
Android is essentially the same as Linux for the purpose of
socket features support, and the Linux kernel has supported
`SO_REUSEPORT` since version 3.9.

According to http://android.stackexchange.com/a/51656,
Android has had more recent kernels than that since version 4.4 Kit Kat.

Author: SimonSapin

src/unix/notbsd/android/mod.rs

@@ -373,6 +373,7 @@ pub const SO_RCVBUF: ::c_int = 8;
 pub const SO_KEEPALIVE: ::c_int = 9;
 pub const SO_OOBINLINE: ::c_int = 10;
 pub const SO_LINGER: ::c_int = 13;
+pub const SO_REUSEPORT: ::c_int = 15;
 pub const SO_RCVLOWAT: ::c_int = 18;
 pub const SO_SNDLOWAT: ::c_int = 19;
 pub const SO_RCVTIMEO: ::c_int = 20;