message: fix header to use u32 instead of u64
diff: @@ -384,14 +384,14 @@ extern "C" {
  extern indy_error_t indy_pack_message(indy_handle_t command_handle,
  indy_handle_t wallet_handle,
  const indy_u8_t* message,
  - indy_u64_t message_len,
  + indy_u32_t message_len,
  const char * receiver_keys,
  const char * sender,
  void (*cb)(indy_handle_t command_handle_,
  indy_error_t err,
  const indy_u8_t* jwe_msg_raw,
  - indy_u64_t jwe_msg_len)
  + indy_u32_t jwe_msg_len)
  );
  @@ -427,12 +427,12 @@ extern "C" {
  extern indy_error_t indy_unpack_message(indy_handle_t command_handle,
  indy_handle_t wallet_handle,
  const indy_u8_t* jwe_msg,
  - indy_u64_t jwe_len,
  + indy_u32_t jwe_len,
  void (*cb)(indy_handle_t command_handle_,
  indy_error_t err,
  const indy_u8_t* res_json_raw,
  - indy_u64_t res_json_len)
  + indy_u32_t res_json_len)
  );
  #ifdef __cplusplus
  }
  