cpu: Provide smpboot_thread_init() on !CONFIG_SMP kernels as well

Now that we are using smpboot_thread_init() in init/main.c as well,
provide it for !CONFIG_SMP as well.

This addresses a !CONFIG_SMP build failure.

Cc: Paul E. McKenney <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

include/linux/cpu.h

···
 {
 }
 
+static inline void smpboot_thread_init(void)
+{
+}
+
 #endif /* CONFIG_SMP */
 extern struct bus_type cpu_subsys;