* companion radio: support for TXT_TYPE_SIGNED_PLAIN

examples/simple_secure_chat/main.cpp

@@ -261,6 +261,8 @@ protected:
 
   void onCommandDataRecv(const ContactInfo& from, uint8_t path_len, uint32_t sender_timestamp, const char *text) override {
   }
+  void onSignedMessageRecv(const ContactInfo& from, uint8_t path_len, uint32_t sender_timestamp, const uint8_t *sender_prefix, const char *text) override {
+  }
 
   void onChannelMessageRecv(const mesh::GroupChannel& channel, int in_path_len, uint32_t timestamp, const char *text) override {
     if (in_path_len < 0) {