Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/arch/x86/xen/vdso.h b/arch/x86/xen/vdso.h
index 861fedf..873c54c 100644
--- a/arch/x86/xen/vdso.h
+++ b/arch/x86/xen/vdso.h
@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
 /* Bit used for the pseudo-hwcap for non-negative segments.  We use
    bit 1 to avoid bugs in some versions of glibc when bit 0 is
    used; the choice is otherwise arbitrary. */