Added missing header in xfixed

Author: tdegeus

include/xtensor/xfixed.hpp

@@ -16,6 +16,8 @@
 #include <utility>
 #include <vector>
 
+#include <xtl/xsequence.hpp>
+
 #include "xcontainer.hpp"
 #include "xstrides.hpp"
 #include "xstorage.hpp"