Add zk_token_sdk_enabled feature to gate Zk Token proof program and sol_zk_token_elgamal_op syscalls

programs/bpf/tests/programs.rs

@@ -484,6 +484,7 @@ fn test_program_bpf_sanity() {
             ("solana_bpf_rust_sanity", true),
             ("solana_bpf_rust_secp256k1_recover", true),
             ("solana_bpf_rust_sha", true),
+            ("solana_bpf_rust_zk_token_elgamal", true),
         ]);
     }