Make pointers to error strings const in TinyXML

The static analyzer (correctly) pointed to the fact that the array of pointers to the error strings in TinyXML kept one from changing the strings but did not keep one from changing the addresses of the pointers in the array. Making the pointers const solves that problem and guarantees immutability.
Merge pull request #5947 from Dr15Jones/makePointersToErrorStringsConstInTinyXML

FWCore/Utilities -- Make pointers to error strings const in TinyXML