spl: Fix compilation with shmem feature enabled

[acheroncrypto]

Problem

anchor-spl crate doesn't compile with all features enabled because shmem feature is broken.

error[E0412]: cannot find type `Pubkey` in this scope
  --> spl/src/shmem.rs:36:10
   |
36 | #[derive(Accounts)]
   |          ^^^^^^^^ not found in this scope
   |
   = help: consider importing one of these items:
           anchor_lang::prelude::Pubkey
           solana_program::example_mocks::solana_sdk::Pubkey
   = note: this error originates in the derive macro `Accounts` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0412]: cannot find type `Pubkey` in this scope
  --> spl/src/shmem.rs:43:10
   |
43 | #[derive(Accounts)]
   |          ^^^^^^^^ not found in this scope
   |
   = help: consider importing one of these items:
           anchor_lang::prelude::Pubkey
           solana_program::example_mocks::solana_sdk::Pubkey
   = note: this error originates in the derive macro `Accounts` (in Nightly builds, run with -Z macro-backtrace for more info)

Summary of changes

Import solana_program::pubkey::Pubkey.

---

Might also remove this feature since the program doesn't seem to be deployed so nobody is using it but let's keep it for now since the fix was easy.

[vercel]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.

[staccDOTsol]

@acheroncrypto I'm finna deploy it maybe

[acheroncrypto]

What is the use case? We removed it in #2747, though it wasn't much anyway.