[llvm] [MC][DecoderEmitter] Fix build warning: explicit specialization cannot have a storage class (PR #156375)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Its weird though that there are just regular pre-commit checks that are failing once I committed the change and why did my PR pass the pre-commit checks.

https://github.com/llvm/llvm-project/pull/156375