fix build, thanks jgarzik

rpc.cpp

@@ -225,8 +225,8 @@ Value gethashespersec(const Array& params, bool fHelp)
             "Returns a recent hashes per second performance measurement while generating.");
 
     if (GetTimeMillis() - nHPSTimerStart > 8000)
-        return (int64)0;
-    return (int64)dHashesPerSec;
+        return (boost::int64_t)0;
+    return (boost::int64_t)dHashesPerSec;
 }