Update to v4.19 kernel headers.

Test: Builds and boots.
Change-Id: I99a9ed79666e143b47f02ca4e59eed94f69b7e4a
(cherry picked from commit a981e2e52e2e95a65fa9c9b6fb16dcb4c83dd576)
diff --git a/libc/kernel/uapi/linux/ip.h b/libc/kernel/uapi/linux/ip.h
index 8632d45..564fd8d 100644
--- a/libc/kernel/uapi/linux/ip.h
+++ b/libc/kernel/uapi/linux/ip.h
@@ -145,6 +145,7 @@
   IPV4_DEVCONF_IGNORE_ROUTES_WITH_LINKDOWN,
   IPV4_DEVCONF_DROP_UNICAST_IN_L2_MULTICAST,
   IPV4_DEVCONF_DROP_GRATUITOUS_ARP,
+  IPV4_DEVCONF_BC_FORWARDING,
   __IPV4_DEVCONF_MAX
 };
 #define IPV4_DEVCONF_MAX (__IPV4_DEVCONF_MAX - 1)