Add CPEPrefix for usrsctp.

This will result in Chrome security bugs being automatically
reported by the Vomit infrastructure for any CVEs reported
against usrsctp.

The version number chosen is the latest version of usrsctp
known by the CVE system, which is a bit imprecise but means
we should get the benefit of any disclosures against the
current version of usrsctp.

See
https://chromium.googlesource.com/chromium/src.git/+/master/docs/adding_to_third_party.md#add-a-readme_chromium
for more details.

Bug: 895969
Change-Id: I389af5824aa9b99c414f78d984aeea95956d692a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427291Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Adrian Taylor <adetaylor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810635}

third_party/usrsctp/README.chromium

 Name: usrsctp
 URL: http://github.com/sctplab/usrsctp
 Version: 0
+CPEPrefix: cpe:/a:usrsctp_project:usrsctp:2019-12-20
 Date: Aug 28, 2020
 Revision: 995c0b84414466d77d52011e5b572cbe213b770a
 License: New BSD