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

gcc-12 with W=1 reports:

 arch/m68k/kernel/machine_kexec.c:26:6: warning: no previous prototype for 'machine_shutdown' [-Wmissing-prototypes]
    26 | void machine_shutdown(void)
       |      ^~~~~~~~~~~~~~~~
 arch/m68k/kernel/machine_kexec.c:30:6: warning: no previous prototype for 'machine_crash_shutdown' [-Wmissing-prototypes]
    30 | void machine_crash_shutdown(struct pt_regs *regs)
       |      ^~~~~~~~~~~~~~~~~~~~~~

No functional changes intended.
Compile tested only.
Signed-off-by: Simon Horman <horms@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20230421-m68k-kexec-include-reboot-v1-1-7552963a0f25@kernel.orgSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>