Add where T : Command constraint on GetCommandsOfType<T>

It doesn't make send to pass anything else but a `Command` as the generic type parameter of `GetCommandsOfType<T>`.

ELFSharp/MachO/MachO.cs

@@ -27,7 +27,7 @@ internal MachO(Stream stream, bool is64, Endianess endianess, bool ownsStream)
             ReadCommands(noOfCommands, stream, reader);
         }
 
-        public IEnumerable<T> GetCommandsOfType<T>()
+        public IEnumerable<T> GetCommandsOfType<T>() where T : Command
         {
             return commands.Where(x => x != null).OfType<T>();
         }