Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/arch/x86/include/asm/bugs.h b/arch/x86/include/asm/bugs.h
index 542509b..794eb21 100644
--- a/arch/x86/include/asm/bugs.h
+++ b/arch/x86/include/asm/bugs.h
@@ -18,4 +18,6 @@
 static inline int ppro_with_ram_bug(void) { return 0; }
 #endif
 
+extern void cpu_bugs_smt_update(void);
+
 #endif /* _ASM_X86_BUGS_H */