regex: omit needless signed-pointer casts

regex: omit needless signed-pointer casts

* lib/regcomp.c (build_charclass, build_charclass_op):
Use char *, not unsigned char *, for class name and extra.
The char values are always nonnegative so there's no need to
insist on unsigned char * here, and using char * removes the need
for casts.  Reported by Aharon Robbins in
<http://sourceware.org/ml/libc-alpha/2012-12/msg00456.html>.