nss - security update

Franziskus Kiefer reported that the existing mitigations for
some timing side-channel attacks were insufficient:
<https://www.mozilla.org/en-US/security/advisories/mfsa2016-90/#&gt;[CVE-2016-9074](https://security-tracker.debian.org/tracker/CVE-2016-9074)

For Debian 7 Wheezy, these problems have been fixed in version
2:3.26-1+debu7u2.

We recommend that you upgrade your nss packages.

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at: <https://wiki.debian.org/LTS&gt;