message: remove "C" from API layer
diff: @@ -636,7 +636,7 @@ pub extern fn indy_pack_message(
  message_len: u32,
  receiver_keys: *const c_char,
  sender: *const c_char,
  - cb: Option<extern "C" fn(xcommand_handle: i32, err: ErrorCode, jwe_data: *const u8, jwe_len: u32)>,
  + cb: Option<extern fn(xcommand_handle: i32, err: ErrorCode, jwe_data: *const u8, jwe_len: u32)>,
  ) -> ErrorCode {
  trace!("indy_pack_message: >>> wallet_handle: {:?}, message: {:?}, message_len {:?},\
  receiver_keys: {:?}, sender: {:?}", wallet_handle, message, message_len, receiver_keys, sender);
  @@ -706,7 +706,7 @@ pub extern fn indy_unpack_message(
  jwe_data: *const u8,
  jwe_len: u32,
  cb: Option<
  - extern "C" fn(
  + extern fn(
  xcommand_handle: i32,
  err: ErrorCode,
  res_json_data : *const u8,
  