gchar is not guaranteed to be signed; use gint8.

C does not guarantee that char is signed (that's why there's a keyword
"signed").  Use gint8, to indicate that it's not a character, it's an
8-bit signed integer.

Change-Id: Id632df23352840cbc86a4d88f4c7dade95ab7837
Reviewed-on: https://code.wireshark.org/review/4736
Reviewed-by: Guy Harris <guy@alum.mit.edu>