gdbstub: Include missing 'hw/core/cpu.h' header

gdbstub: Include missing 'hw/core/cpu.h' header

Functions such gdb_get_cpu_pid() dereference CPUState so
require the structure declaration from "hw/core/cpu.h":

  static uint32_t gdb_get_cpu_pid(CPUState *cpu)
  {
    ...
    return cpu->cluster_index + 1;
  }

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20231211212003.21686-15-philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>