provide attribution for protobuf-c

The agent uses protobuf-c library to provide support infinite tracing - infinite
tracing endpoint speaks protobuf.

Author: lavarou

THIRD_PARTY_NOTICES.md

@@ -23,6 +23,7 @@ by e-mailing [[email protected]](mailto:[email protected]).
 * [PCRE](#pcre)
 * [PHP](#php)
 * [Portable OpenSSH](#portable-openssh)
+* [protobuf-c](#protobuf-c)
 * [TSRM](#tsrm)
 * [Symfony](#symfony)
 
@@ -895,6 +896,46 @@ under
  */
 ```
 
+## protobuf-c
+
+This product includes [protobuf-c]https://github.com/protobuf-c/protobuf-c) ([v1.3.3](https://github.com/protobuf-c/protobuf-c/tree/v1.3.3)),
+distributed under the [protobuf-c license](https://github.com/protobuf-c/protobuf-c/blob/v1.3.3/LICENSE):
+
+```
+Copyright (c) 2008-2016, Dave Benson and the protobuf-c authors.
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+    * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+The code generated by the protoc-gen-c code generator and by the
+protoc-c compiler is owned by the owner of the input files used when
+generating it. This code is not standalone and requires a support
+library to be linked with it. This support library is covered by the
+above license.
+```
+
 ## TSRM
 
 This product includes source derived from