zebra, isisd: cast to unsigned char for ctypes

zebra, isisd: cast to unsigned char for ctypes

ctype.h macros take int as arguments, but expect arguments to be in
unsigned char's range.  Even though it probably works, this isn't
correct on systems that have a signed char type.  Cast explicitly.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 52f02b47685bc823c4c75560175a27aab0bd6709)