api: use weak imports for plugin protobufs

Because of a side-effect import, we have the possibility of pulling in
several unnecessary packages that are used by the plugin and not at
runtime to implement protobuf structures. Setting these imports to
`weak` prevents this from happening, reducing the total import set,
reducing memory usage and binary size.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

linux/runctypes/next.pb.txt

@@ -178,5 +178,6 @@ file {
   options {
     go_package: "github.com/containerd/containerd/linux/runctypes;runctypes"
   }
+  weak_dependency: 0
   syntax: "proto3"
 }