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/manifest

@@ -155,9 +155,11 @@
 		{
 			"importpath": "github.com/gogo/protobuf",
 			"repository": "https://github.com/gogo/protobuf",
-			"vcs": "",
-			"revision": "7883e1468d48d969e1c3ce4bcde89b6a7dd4adc4",
-			"branch": "master"
+			"vcs": "git",
+			"revision": "f6b4bb7b2dde1736b809b3da996ed72f278e9be9",
+			"branch": "master",
+			"notests": true,
+			"allfiles": true
 		},
 		{
 			"importpath": "github.com/golang/groupcache/lru",