value: Use a signed char to old g_value_get_schar result

char and signed char are the same in most of platforms, but that's not always true such as in arm64, s390x, ppc64el and others.

See: https://buildd.debian.org/status/fetch.php?pkg=gjs&arch=arm64&ver=1.75.2-1&stamp=1676991213&raw=0

Closes: #529 (closed)