make Ubuntu 16.04 happy

src/engine/fileOps/tfm.cpp

@@ -172,13 +172,14 @@ struct TFMSpeed {
 };
 
 // to make it work with map
-template<>
-struct std::hash<TFMSpeed>
+namespace std {
+  template<> struct hash<TFMSpeed>
   {
     size_t operator()(const TFMSpeed& s) const noexcept {
       return s.speedEven<<16|s.speedOdd<<8|s.interleaveFactor;
     }
   };
+}
 
 struct TFMParsePatternInfo {
   TFMRLEReader* reader;