Update to v4.15 kernel headers.

Test: Compiles, boots bullhead/hikey960.
Change-Id: I118beb8b6cac0881b1270f9bf6981959297a41a8
diff --git a/libc/kernel/uapi/linux/in6.h b/libc/kernel/uapi/linux/in6.h
index 1e3b49d..9da111c 100644
--- a/libc/kernel/uapi/linux/in6.h
+++ b/libc/kernel/uapi/linux/in6.h
@@ -175,4 +175,5 @@
 #define IPV6_TRANSPARENT 75
 #define IPV6_UNICAST_IF 76
 #define IPV6_RECVFRAGSIZE 77
+#define IPV6_FREEBIND 78
 #endif