Fix compiler warning

[Ulrich Drepper]

"H . J . Lu" <hjl@lucon.org> writes:

> Why not (void *) which is used in prototype?

Mark copied only what I did in similar places (or at least I can
explain it this way).  I prefer char* here since it shows that this is
actually a modifiable string.  If I use a cast to void* I normally do
something much dirtier, like casting a long to a pointer.

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