Make eval_plan const

Enclave/sort.cpp

@@ -108,7 +108,7 @@ void sort_mtobliv(threadid_t nthreads, uint8_t* items, uint16_t msg_size,
             Nw, nthreads);
         assert(false);
     }
-    WNEvalPlan &eval_plan = precomp_eps.eval_plans.at(epidx);
+    const WNEvalPlan &eval_plan = precomp_eps.eval_plans.at(epidx);
     pthread_mutex_unlock(&precomp_eps.mutex);
     // Mark Nw-Nr items as padding (Nr, Na, and Nw are _not_ private)
     for (uint32_t i=Nr; i<Nw; ++i) {