sstatesig.py: fix logic in find_siginfo

sstatesig.py: fix logic in find_siginfo

For now, `bitbake-diffsig -t <recipe> <task>' doesn't work. This is
caused by a small logic mistake in find_siginfo in sstatesig.py.

The logic should be 'and' instead of 'or', otherwise, we will have
both siginfo and sigdata files in filedates which have the same checksum.
e.g.
/buildarea2/chenqi/sstate-cache/fc/sstate:sysstat:armv5te-poky-linux-gnueabi:10.2.1:r0:armv5te:3:fc861bf371c1b843b2843a3415eb5ff3_install.tgz.siginfo
/buildarea2/chenqi/poky/build-systemd/tmp/stamps/armv5te-poky-linux-gnueabi/sysstat/10.2.1-r0.do_install.sigdata.fc861bf371c1b843b2843a3415eb5ff3

So `bitbake-diffsig -t sysstat install' will output nothing even we actually
have changed something in do_install task.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>