Fix build on Android

Author: asomers

src/unistd.rs

@@ -974,6 +974,9 @@ CHILD_MAX = libc::_SC_CHILD_MAX,
 COLL_WEIGHTS_MAX = libc::_SC_COLL_WEIGHTS_MAX,
 DELAYTIMER_MAX = libc::_SC_DELAYTIMER_MAX,
 EXPR_NEST_MAX = libc::_SC_EXPR_NEST_MAX,
+#[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
+          target_os="linux", target_os = "macos", target_os="netbsd",
+          target_os="openbsd"))]
 HOST_NAME_MAX = libc::_SC_HOST_NAME_MAX,
 IOV_MAX = libc::_SC_IOV_MAX,
 LINE_MAX = libc::_SC_LINE_MAX,