arm64: smp: Make cpus_stuck_in_kernel static

Fix the following sparse warning:

arch/arm64/kernel/smp.c:68:5: warning: symbol 'cpus_stuck_in_kernel'
was not declared. Should it be static?

Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Zou Wei <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/kernel/smp.c

···
  */
 struct secondary_data secondary_data;
 /* Number of CPUs which aren't online, but looping in kernel text. */
-int cpus_stuck_in_kernel;
+static int cpus_stuck_in_kernel;
 
 enum ipi_msg_type {
 	IPI_RESCHEDULE,