valgrind.supp: ignore libnss3 leaks

These just started popping up when I updated the notcmalloc gitbuilder, probably
because of an updated libnss version.   Whitelist it!

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 9a17bb5134ae7072089f780cd975761e61c708db)