Don't include i18n in headers

The i18n header needs to be included last, since it defines
`_` which results in conflicts with other headers.

see #8361 and e.g.
boost/function_types/detail/class_transform.hpp:23:26:
error: ‘boost::mpl::placeholders::_’ has not been declared
using mpl::placeholders::_;

libs/ardour/ardour/types_convert.h

@@ -25,7 +25,6 @@
 #endif
 
 #include "pbd/enum_convert.h"
-#include "pbd/i18n.h"
 
 #include "ardour/types.h"
 #include "ardour/data_type.h"