Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/tools/build/feature/test-setns.c b/tools/build/feature/test-setns.c
index 1f714d2..2757c20 100644
--- a/tools/build/feature/test-setns.c
+++ b/tools/build/feature/test-setns.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 #define _GNU_SOURCE
 #include <sched.h>
 
@@ -5,3 +6,4 @@
 {
 	return setns(0, 0);
 }
+#undef _GNU_SOURCE