xtensa: traps: add <linux/cpu.h> for function prototype

Use <linux/cpu.h> to provide the prototype for trap_init(), to prevent
a build warning:

arch/xtensa/kernel/traps.c:484:13: warning: no previous prototype for 'trap_init' [-Wmissing-prototypes]
  484 | void __init trap_init(void)

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230920052139.10570-9-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>