xtensa: signal: include headers for function prototypes

Add <asm/syscall.h> to satisfy the xtensa_rt_sigreturn() prototype
warning.
Add <asm/processor.h> to satisfy the do_notify_resume() prototype
warning.

arch/xtensa/kernel/signal.c:246:17: warning: no previous prototype for 'xtensa_rt_sigreturn' [-Wmissing-prototypes]
arch/xtensa/kernel/signal.c:525:6: warning: no previous prototype for 'do_notify_resume' [-Wmissing-prototypes]
  525 | void do_notify_resume(struct pt_regs *regs)

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230920052139.10570-7-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>