Upgrade to new NSS PKCS#11 api change

[pabuhler]

This is a follow from the quick fix #524 for #523. There was change in NSS 3.52 that breaks GCM when the api is used in the same way as before. https://bugzilla.mozilla.org/show_bug.cgi?id=1637488#c3 contains more info and possible options to mitigate this. Currently #524 was the implementation of option 1 but should consider making a more complete fix based on one of the other alternatives.

The best would be to find a solution that does not place any unnecessary burden on users of libsrtp.

PR's welcome!