fastly: fix lifetime warning

```
error: elided lifetime has a name
   --> deps/rust-rdkafka/src/admin.rs:914:83
    |
902 | impl<'a> NewPartitions<'a> {
    |      -- lifetime `'a` declared here
...
914 |     pub fn assign(mut self, assignment: PartitionAssignment<'a>) -> NewPartitions<'_> {
    |                                                                                   ^^ this elided lifetime gets resolved as `'a`
    |
    = note: `-D elided-named-lifetimes` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(elided_named_lifetimes)]`
```

Author: sharksforarms

src/admin.rs

@@ -911,7 +911,7 @@ impl<'a> NewPartitions<'a> {
 
     /// Sets the partition replica assignment for the new partitions. Only
     /// assignments for newly created replicas should be included.
-    pub fn assign(mut self, assignment: PartitionAssignment<'a>) -> NewPartitions<'_> {
+    pub fn assign(mut self, assignment: PartitionAssignment<'a>) -> NewPartitions<'a> {
         self.assignment = Some(assignment);
         self
     }