Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/linux/if_tunnel.h b/libc/kernel/uapi/linux/if_tunnel.h
index bfc6479..8edc956 100644
--- a/libc/kernel/uapi/linux/if_tunnel.h
+++ b/libc/kernel/uapi/linux/if_tunnel.h
@@ -93,6 +93,7 @@
   TUNNEL_ENCAP_NONE,
   TUNNEL_ENCAP_FOU,
   TUNNEL_ENCAP_GUE,
+  TUNNEL_ENCAP_MPLS,
 };
 #define TUNNEL_ENCAP_FLAG_CSUM (1 << 0)
 #define TUNNEL_ENCAP_FLAG_CSUM6 (1 << 1)