Update to v4.19 kernel headers.

Test: Builds and boots.
Change-Id: I99a9ed79666e143b47f02ca4e59eed94f69b7e4a
diff --git a/libc/kernel/uapi/linux/signalfd.h b/libc/kernel/uapi/linux/signalfd.h
index a1fe2de..5108ef0 100644
--- a/libc/kernel/uapi/linux/signalfd.h
+++ b/libc/kernel/uapi/linux/signalfd.h
@@ -40,6 +40,10 @@
   __u64 ssi_stime;
   __u64 ssi_addr;
   __u16 ssi_addr_lsb;
-  __u8 __pad[46];
+  __u16 __pad2;
+  __s32 ssi_syscall;
+  __u64 ssi_call_addr;
+  __u32 ssi_arch;
+  __u8 __pad[28];
 };
 #endif