Include reboot.h in machine_kexec.c for declaration of
machine_crash_shutdown.

gcc-12 with W=1 reports:

 arch/arm64/kernel/machine_kexec.c:257:6: warning: no previous prototype for 'machine_crash_shutdown' [-Wmissing-prototypes]
   257 | void machine_crash_shutdown(struct pt_regs *regs)

No functional changes intended.
Compile tested only.
Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230418-arm64-kexec-include-reboot-v1-1-8453fd4fb3fb@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>