Make sure to include zlib.BUILD in our distribution

Makefile.am

@@ -1223,6 +1223,7 @@ EXTRA_DIST = $(@DIST_LANG@_EXTRA_DIST)   \
   python/release/wheel/protobuf_optimized_pip.sh \
   python/release/wheel/README.md         \
   six.BUILD                              \
+  third_party/zlib.BUILD                 \
   util/python/BUILD