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

gcc-12 with W=1 reports:

 arch/parisc/kernel/kexec.c:57:6: warning: no previous prototype for 'machine_crash_shutdown' [-Wmissing-prototypes]
    57 | void machine_crash_shutdown(struct pt_regs *regs)
       |      ^~~~~~~~~~~~~~~~~~~~~~
 arch/parisc/kernel/kexec.c:61:6: warning: no previous prototype for 'machine_shutdown' [-Wmissing-prototypes]
    61 | void machine_shutdown(void)
       |      ^~~~~~~~~~~~~~~~

No functional changes intended.
Compile tested only.
Signed-off-by: Simon Horman <horms@kernel.org>
Acked-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Helge Deller <deller@gmx.de>