lib/oe/sstatesig: fix wildcard matching wrong task signature files

lib/oe/sstatesig: fix wildcard matching wrong task signature files

With a '*' as a wildcard for the signature here we can also match a
portion of the task name with the result that we may match a sigdata
file for the wrong task. Luckily the signature is always the same
length - 32 characters - so we can simply use 32 '?' characters instead.
(A regex would have been another alternative, but the wildcard should be
effective and I felt like a regex would complicate the code more than
this solution).

Fixes [YOCTO #11763].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>