Update to v4.19 kernel headers.

Test: Builds and boots.
Change-Id: I99a9ed79666e143b47f02ca4e59eed94f69b7e4a
(cherry picked from commit a981e2e52e2e95a65fa9c9b6fb16dcb4c83dd576)
diff --git a/libc/kernel/uapi/asm-generic/socket.h b/libc/kernel/uapi/asm-generic/socket.h
index b821554..7c52ccc 100644
--- a/libc/kernel/uapi/asm-generic/socket.h
+++ b/libc/kernel/uapi/asm-generic/socket.h
@@ -88,4 +88,6 @@
 #define SCM_TIMESTAMPING_PKTINFO 58
 #define SO_PEERGROUPS 59
 #define SO_ZEROCOPY 60
+#define SO_TXTIME 61
+#define SCM_TXTIME SO_TXTIME
 #endif