Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/linux/ipv6.h b/libc/kernel/uapi/linux/ipv6.h
index d07fa06..ff345a6 100644
--- a/libc/kernel/uapi/linux/ipv6.h
+++ b/libc/kernel/uapi/linux/ipv6.h
@@ -140,6 +140,7 @@
   DEVCONF_ADDR_GEN_MODE,
   DEVCONF_DISABLE_POLICY,
   DEVCONF_ACCEPT_RA_RT_INFO_MIN_PLEN,
+  DEVCONF_NDISC_TCLASS,
   DEVCONF_MAX
 };
 #endif