fix clang compilation (include header ordering)

"i18n.h" needs to be included last, after types have been defined or
clang bails out:
"call to function 'operator<<' is not visible in the template definition"

libs/ardour/disk_io.cc

@@ -19,7 +19,6 @@
 
 #include "pbd/debug.h"
 #include "pbd/error.h"
-#include "pbd/i18n.h"
 
 #include "ardour/audioplaylist.h"
 #include "ardour/butler.h"
@@ -36,6 +35,8 @@
 #include "ardour/session.h"
 #include "ardour/session_playlists.h"
 
+#include "pbd/i18n.h"
+
 using namespace ARDOUR;
 using namespace PBD;
 using namespace std;

libs/ardour/disk_reader.cc

@@ -18,7 +18,6 @@
 */
 
 #include "pbd/enumwriter.h"
-#include "pbd/i18n.h"
 #include "pbd/memento_command.h"
 
 #include "ardour/audioengine.h"
@@ -36,6 +35,8 @@
 #include "ardour/session.h"
 #include "ardour/session_playlists.h"
 
+#include "pbd/i18n.h"
+
 using namespace ARDOUR;
 using namespace PBD;
 using namespace std;

libs/ardour/disk_writer.cc

@@ -17,8 +17,6 @@
 
 */
 
-#include "pbd/i18n.h"
-
 #include "ardour/analyser.h"
 #include "ardour/audioengine.h"
 #include "ardour/audiofilesource.h"
@@ -36,6 +34,8 @@
 #include "ardour/session.h"
 #include "ardour/smf_source.h"
 
+#include "pbd/i18n.h"
+
 using namespace ARDOUR;
 using namespace PBD;
 using namespace std;