Update to kernel headers v4.12.3.

Test: Built angler.
Change-Id: Icbcf4fac2334de8409b049ed7a3b4c24b4e98ce9
diff --git a/libc/kernel/uapi/linux/mpls_iptunnel.h b/libc/kernel/uapi/linux/mpls_iptunnel.h
index 773a49f..473cb0d 100644
--- a/libc/kernel/uapi/linux/mpls_iptunnel.h
+++ b/libc/kernel/uapi/linux/mpls_iptunnel.h
@@ -21,6 +21,7 @@
 enum {
   MPLS_IPTUNNEL_UNSPEC,
   MPLS_IPTUNNEL_DST,
+  MPLS_IPTUNNEL_TTL,
   __MPLS_IPTUNNEL_MAX,
 };
 #define MPLS_IPTUNNEL_MAX (__MPLS_IPTUNNEL_MAX - 1)