Suppress warnings about conversions between char * and unsigned char *.

For some time PSPP has used "unsigned char *" for string data and "char *"
for most other strings.  Transforming between these properly generally
involves recoding, and there are some places where this is possibly
missing.  The usual way to "fix" these warnings would be to insert casts,
but this would not fix the problem, just suppress the warnings, and it is
difficult to "grep" for casts so they would be hard to find for fixing
later.

This commit doesn't actually fix the problems, but it does suppress the
warnings while making them easy to find later: just grep for CHAR_CAST_BUG.