hexagon: process: include linux/cpu.h for arch_cpu_idle() prototype

Clang warns:

  arch/hexagon/kernel/process.c:43:6: warning: no previous prototype for function 'arch_cpu_idle' [-Wmissing-prototypes]
     43 | void arch_cpu_idle(void)
        |      ^
  arch/hexagon/kernel/process.c:43:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
     43 | void arch_cpu_idle(void)
        | ^
        | static

This prototype is declared in include/linux/cpu.h, include it in
process.c to clear up the warning.

Link: https://lkml.kernel.org/r/20231130-hexagon-missing-prototypes-v1-13-5c34714afe9e@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>