hexagon: reset: include linux/reboot.h for prototypes

Clang warns about missing prototypes that are declared in this header:

  arch/hexagon/kernel/reset.c:9:6: warning: no previous prototype for function 'machine_power_off' [-Wmissing-prototypes]
      9 | void machine_power_off(void)
        |      ^
  arch/hexagon/kernel/reset.c:9:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
      9 | void machine_power_off(void)
        | ^
        | static
  arch/hexagon/kernel/reset.c:15:6: warning: no previous prototype for function 'machine_halt' [-Wmissing-prototypes]
     15 | void machine_halt(void)
        |      ^
  arch/hexagon/kernel/reset.c:15:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
     15 | void machine_halt(void)
        | ^
        | static
  arch/hexagon/kernel/reset.c:19:6: warning: no previous prototype for function 'machine_restart' [-Wmissing-prototypes]
     19 | void machine_restart(char *cmd)
        |      ^
  arch/hexagon/kernel/reset.c:19:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
     19 | void machine_restart(char *cmd)
        | ^
        | static
  3 warnings generated.

Link: https://lkml.kernel.org/r/20231130-hexagon-missing-prototypes-v1-12-5c34714afe9e@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Brian Cain <bcain@quicinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>