x86/cpu: Add a missing prototype for arch_smt_update()

.. in order to fix a -Wmissing-prototype warning.

No functional change.
Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200109121723.8151-1-b.thiel@posteo.de

arch/x86/kernel/cpu/common.c

@@ -14,6 +14,7 @@
 #include <linux/sched/mm.h>
 #include <linux/sched/clock.h>
 #include <linux/sched/task.h>
+#include <linux/sched/smt.h>
 #include <linux/init.h>
 #include <linux/kprobes.h>
 #include <linux/kgdb.h>