base: Fix -Werror=unused-variable in Memoizer

Change-Id: I222a445121a35a4abc1a3fa2068e6c26ad7ee843
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59429
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/base/memoizer.hh

@@ -103,8 +103,8 @@ class Memoizer
             // Return the cached result
             return it->second;
         }
-        auto [it, emplaced] = cache.emplace(key, func(args...));
-        return it->second;
+        auto emplaced = cache.emplace(key, func(args...));
+        return emplaced.first->second;
     };
 
     /** Clear the memoization cache */