Fix compilation error (#3357)

Address #3356

## Motivation and Context
Solves compilation error mentioned in #3356 

## Description
Qualify access to `base64` module

## Testing
Trivial compilation changes has been done - `cargo build` succeeds with
this change, while it doesn't without it. Provided of course that
`aws_sdk_unstable` config is selected and `serde-serialize` feature is
enabled.

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