[Bug cobol/122398] [16 regression] libgcobol fails to build w/ libxml2-2.12.0

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122398

--- Comment #3 from Sam James <sjames at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #2)
> so simply make msg const I guess.

I think that may fail with older libxml2, so some ifdefery is needed. But yeah,
that's what I've done as a hack.