Compile fix.

libethereum/BlockChain.h

@@ -42,10 +42,13 @@
 #include "BlockQueue.h"
 namespace ldb = leveldb;
 
-template <> struct std::hash<std::pair<dev::h256, unsigned>>
+namespace std
 {
-	size_t operator()(const pair<dev::h256, unsigned> &x ) const { return std::hash<dev::h256>()(x.first) ^ std::hash<unsigned>()(x.second); }
+template <> struct hash<pair<dev::h256, unsigned>>
+{
+	size_t operator()(pair<dev::h256, unsigned> const& _x) const { return hash<dev::h256>()(_x.first) ^ hash<unsigned>()(_x.second); }
 };
+}
 
 namespace dev
 {