Adding zlib LICENSE to generate_licenses.py.

Starting from https://webrtc-review.googlesource.com/c/src/+/18140
(which includes 3296e11b37)
//third_party/protobuf starts to depend on //third_party/zlib.

To fix the Chromium Roll WebRTC has add the license file of
//third_party/zlib to its generate_licenses.py script in order to add
it to markdown generated license file.

Bug: None
Change-Id: If504ef00b166fdbcbe22acb0a2721bfb55624d3e
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/18244
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20566}

tools_webrtc/libs/generate_licenses.py

@@ -40,6 +40,7 @@ LIB_TO_LICENSES_DICT = {
     'protobuf': ['third_party/protobuf/LICENSE'],
     'usrsctp': ['third_party/usrsctp/LICENSE'],
     'webrtc': ['LICENSE', 'LICENSE_THIRD_PARTY'],
+    'zlib': ['LICENSE', 'third_party/zlib/LICENSE'],
 
     # Compile time dependencies, no license needed:
     'yasm': [],