sstatesig: Fix locked signature handling with unihashes
get_taskhash will call get_unihash internally in the parent class. We
need to disable our filter of it whilst this runs else incorrect hashes
can be calculated.
This is believed to be causing the locked signatures test to fail under
some circumstances (depending on whether earlier hashes are being
remapped).
[YOCTO #13605]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
