metag/smp: Make boot_secondary() static

boot_secondary() is not used outside arch/metag/kernel/smp.c, hence make it
static.
Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: James Hogan <james.hogan@imgtec.com>

arch/metag/kernel/smp.c

@@ -68,7 +68,7 @@ static DECLARE_COMPLETION(cpu_running);
 /*
  * "thread" is assumed to be a valid Meta hardware thread ID.
  */
-int boot_secondary(unsigned int thread, struct task_struct *idle)
+static int boot_secondary(unsigned int thread, struct task_struct *idle)
 {
 	u32 val;