vendor: Update github.com/gogo/protobuf

Also tweaks the proto definitions:

 - [packed=false] on the block_indexes field to retain compat with
   v0.14.16 and earlier.

 - Uses the vendored protobuf package in include paths.

And, "build.go setup" will install the vendored protoc-gen-gogofast.
This should ensure that a proto rebuild isn't so dependent on whatever
version of the compiler and package the developer has installed...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3864

vendor/github.com/gogo/protobuf/AUTHORS

@@ -0,0 +1,14 @@
+# This is the official list of GoGo authors for copyright purposes.
+# This file is distinct from the CONTRIBUTORS file, which
+# lists people.  For example, employees are listed in CONTRIBUTORS,
+# but not in AUTHORS, because the employer holds the copyright.
+
+# Names should be added to this file as one of
+#     Organization's name
+#     Individual's name <submission email address>
+#     Individual's name <submission email address> <email2> <emailN>
+
+# Please keep the list sorted.
+
+Vastech SA (PTY) LTD
+Walter Schulze <awalterschulze@gmail.com>