Suppress compiler warnings when building with --enable-dtrace.

Suppress compiler warnings when building with --enable-dtrace.

Most versions of "dtrace -h" drop const qualifiers from the declarations
of probe functions (though macOS gets it right).  This causes compiler
warnings when we pass in pointers to const.  Repair by extending our
existing post-processing of the probes.h file.  To do so, assume that all
"char *" arguments should be "const char *"; that seems reasonably safe.

Thomas Munro

Discussion: https://postgr.es/m/CAEepm=2j1pWSruQJqJ91ZDzD8w9ZZDsM4j2C6x75C-VryWg-_w@mail.gmail.com