Set a go_package on api/types/swarm/runtime to make it Bazel compatible

Signed-off-by: Yves Junqueira <yves.junqueira@gmail.com>

api/types/swarm/runtime/plugin.proto

@@ -1,5 +1,7 @@
 syntax = "proto3";
 
+option go_package = "github.com/docker/docker/api/types/swarm/runtime;runtime";
+
 // PluginSpec defines the base payload which clients can specify for creating
 // a service with the plugin runtime.
 message PluginSpec {