semihosting: Include missing 'gdbstub/syscalls.h' header

semihosting: Include missing 'gdbstub/syscalls.h' header

"semihosting/syscalls.h" requires definitions from
"gdbstub/syscalls.h", include it in order to avoid:

  include/semihosting/syscalls.h:23:38: error: unknown type name 'gdb_syscall_complete_cb'
  void semihost_sys_open(CPUState *cs, gdb_syscall_complete_cb complete,
                                       ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240717105723.58965-2-philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240718094523.1198645-9-alex.bennee@linaro.org>