error: invalid operands of types 'const char [13]' and 'const char*' to binary 'operator+'

[szprytny]

This is compilation time error when build_flags contain WEBSERIAL_DEBUG flag

https://github.com/ayushsharma82/WebSerial/blob/97d78305334b4117e9c53433c5ff11ef8db42a3c/src/WebSerial.cpp#L58