Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/linux/seg6.h b/libc/kernel/uapi/linux/seg6.h
index 6f7b8dd..f180485 100644
--- a/libc/kernel/uapi/linux/seg6.h
+++ b/libc/kernel/uapi/linux/seg6.h
@@ -27,7 +27,7 @@
   __u8 segments_left;
   __u8 first_segment;
   __u8 flags;
-  __u16 reserved;
+  __u16 tag;
   struct in6_addr segments[0];
 };
 #define SR6_FLAG1_PROTECTED (1 << 6)