arm64/process.c: fix Wmissing-prototypes build warnings

Fix GCC warnings reported when building with "-Wmissing-prototypes":

arch/arm64/kernel/process.c:261:6: warning: no previous prototype for '__show_regs' [-Wmissing-prototypes]
261 | void __show_regs(struct pt_regs *regs)
| ^~~~~~~~~~~
arch/arm64/kernel/process.c:307:6: warning: no previous prototype for '__show_regs_alloc_free' [-Wmissing-prototypes]
307 | void __show_regs_alloc_free(struct pt_regs *regs)
| ^~~~~~~~~~~~~~~~~~~~~~
arch/arm64/kernel/process.c:365:5: warning: no previous prototype for 'arch_dup_task_struct' [-Wmissing-prototypes]
365 | int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
| ^~~~~~~~~~~~~~~~~~~~
arch/arm64/kernel/process.c:546:41: warning: no previous prototype for '__switch_to' [-Wmissing-prototypes]
546 | __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
| ^~~~~~~~~~~
arch/arm64/kernel/process.c:710:25: warning: no previous prototype for 'arm64_preempt_schedule_irq' [-Wmissing-prototypes]
710 | asmlinkage void __sched arm64_preempt_schedule_irq(void)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~

Link: https://lore.kernel.org/lkml/[e-mail]
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Maninder Singh <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/include/asm/processor.h

···
 extern struct task_struct *cpu_switch_to(struct task_struct *prev,
 					 struct task_struct *next);
 
+asmlinkage void arm64_preempt_schedule_irq(void);
+
 #define task_pt_regs(p) \
 	((struct pt_regs *)(THREAD_SIZE + task_stack_page(p)) - 1)
 

arch/arm64/include/asm/thread_info.h

···
 #define arch_setup_new_exec     arch_setup_new_exec
 
 void arch_release_task_struct(struct task_struct *tsk);
+int arch_dup_task_struct(struct task_struct *dst,
+				struct task_struct *src);
 
 #endif
 

arch/arm64/kernel/process.c

···
 #include <asm/processor.h>
 #include <asm/pointer_auth.h>
 #include <asm/stacktrace.h>
+#include <asm/switch_to.h>
+#include <asm/system_misc.h>
 
 #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
 #include <linux/stackprotector.h>