Fix ChunkFile for std::set

Without this, attempts to savestate std::set will fail with an error
about dropping the const qualifier.

<Lioncash> leoetlino: I'll try to break it down: So, when you do a
 ranged-for on a container, it's essentially syntactic sugar over begin
 and end iterators. std::set is an associative container where the key
 type is the same as the value type, and so it's required that all
 iterator functions return constant iterators. If this wasn't a
 requirement, it would allow changing the ordering of elements from
 outside of the set's API (this is bad).

Source/Core/Common/ChunkFile.h

@@ -120,7 +120,7 @@ public:
     case MODE_WRITE:
     case MODE_MEASURE:
     case MODE_VERIFY:
-      for (V& val : x)
+      for (const V& val : x)
       {
         Do(val);
       }