add miniutf to 3rd party license file (DataDog#296)

Author: lucaspimentel

LICENSE-3rdparty.csv

@@ -7,3 +7,4 @@ clr-samples,https://github.com/Microsoft/clr-samples,MIT,Copyright (c) .NET Foun
 clrprofiler,https://github.com/MicrosoftArchive/clrprofiler,MIT,Copyright (c) Microsoft Corporation.  All rights reserved.
 JSON for Modern C++,https://github.com/nlohmann/json,MIT,Copyright (c) 2013-2018 Niels Lohmann
 Sigil,https://github.com/kevin-montrose/Sigil,MS-PL,2013-2016 Kevin Montrose
+miniutf,https://github.com/dropbox/miniutf,MIT,"Copyright (c) 2013 Dropbox, Inc."