Note known failures for TCP_USER_TIMEOUT tests on host

TCP_USER_TIMEOUT is not currently defined on the prebuilt tcp.h (at least
not on x86-64), so its value at the non-native level remains 0, breaking
tests that rely on it.

Devices kernels and bionic support it, therefore these tests are only
expected to break on host.

Test: Nothing. Buildbot will let us know soon enough if the expected
      failures do not match reality.

Bug: 30402085
Change-Id: Iacaf63ab341c6e4a41c7d21fb3064d58ada94800
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index bf8d12b..b06d7c6 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -36,6 +36,15 @@
   names: ["libcore.io.OsTest#testUnixDomainSockets_in_file_system"]
 },
 {
+  description: "TCP_USER_TIMEOUT is not defined on host's tcp.h (glibc-2.15-4.8).",
+  result: EXEC_FAILED,
+  modes: [host],
+  names: ["libcore.android.system.OsConstantsTest#testTcpUserTimeoutIsDefined",
+          "libcore.io.OsTest#test_socket_tcpUserTimeout_setAndGet",
+          "libcore.io.OsTest#test_socket_tcpUserTimeout_doesNotWorkOnDatagramSocket"],
+  bug: 30402085
+},
+{
   description: "Issue with incorrect device time (1970)",
   result: EXEC_FAILED,
   modes: [device],