regexp.h: work around -Werror compile failure of XS in linux perf tool

regexp.h: work around -Werror compile failure of XS in linux perf tool

The linux perf tool has an XS component, but when built using system
perl 5.14.0, its compilation errors out on the switch statement in
regexp.h: get_regex_charset_name(), which lacks a default case.  Add
one, copying the end-of-function return "?".

Preserve the end-of-function return "?", to avoid a hypothetical
compiler bug which misses the default case, and infers a void return
for a function thats declared otherwize.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>