devel/nspr: update to 4.15

NSPR doesn't support TCP Fast Open on BSDs (patches welcome).
See https://bugzilla.mozilla.org/show_bug.cgi?id=1188435 for
how www/firefox is going to use it.

Changes: http://mozilla.6506.n7.nabble.com/ANNOUNCE-NSPR-4-15-Release-td365835.html
ABI: https://abi-laboratory.pro/tracker/timeline/nspr/
MFH: 2017Q2