Bug 1520103 - Added missing includes in nsNativeCharsetUtils.h - r=froydnj

Found when forcing a non-unified build of the Gecko Profiler:
- "nsError.h" needed to define `nsresult`.
- "nsStringFwd.h" needed to declare `nsAString` and `nsACString`.

Differential Revision: https://phabricator.services.mozilla.com/D18621

--HG--
extra : moz-landing-system : lando

xpcom/io/nsNativeCharsetUtils.h

@@ -23,6 +23,9 @@
  *                                                                           *
 \*****************************************************************************/
 
+#include "nsError.h"
+#include "nsStringFwd.h"
+
 /**
  * thread-safe conversion routines that do not depend on uconv libraries.
  */