cpu: Make hash struct instead of class to please clang

This patch changes the type of the hash function for BasicBlockRanges
to match the original definition of the templatized type. Without
this, clang raises a warning and combined with the "-Werror" flag this
causes compilation to fail.

src/cpu/simple/atomic.hh

@@ -58,7 +58,7 @@ typedef std::pair<Addr, Addr> BasicBlockRange;
 /** Overload hash function for BasicBlockRange type */
 __hash_namespace_begin
 template <>
-class hash<BasicBlockRange>
+struct hash<BasicBlockRange>
 {
   public:
     size_t operator()(const BasicBlockRange &bb) const {