lib: Improve the is_signed_type() kunit test

Since the definition of is_signed_type() has been moved from
<linux/overflow.h> to <linux/compiler.h>, include the latter header file
instead of the former. Additionally, add a test for the type 'char'.

Cc: Isabella Basso <isabbasso@riseup.net>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220907180329.3825417-1-bvanassche@acm.org