`constexpr` doesn't imply `const`?!?

This silences this warning:

```
.....libraries/SNMP/src/SNMPMessage.h:127:53: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
  127 |         static constexpr char *SNMPTRAPENTERPRISE = "1.3.6.1.6.3.1.1.4.3.0";
      |                                                     ^~~~~~~~~~~~~~~~~~~~~~~
```