libedit: Disable -Wuse-after-free for chartype.c.

GCC 12 thinks ct_visual_string can reuse a pointer after it has been
reallocated, but in this case the warning appears false.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D36820

(cherry picked from commit 7973f26ad6f98e6da3e2bb07a1015270c0d16c43)