cbuf: allow inheritance (#5883)

changes "private" vars to "protected" so that descendants are allowed to access them.

cores/esp32/cbuf.h

@@ -62,7 +62,7 @@ public:
 
     cbuf *next;
 
-private:
+protected:
     inline char* wrap_if_bufend(char* ptr) const
     {
         return (ptr == _bufend) ? _buf : ptr;