Resource Editor: Add missing Q_OBJECT to  PrefixLangDialog.

src/plugins/resourceeditor/resourceeditorplugin.cpp:74: Class 'PrefixLangDialog' lacks Q_OBJECT macro

Change-Id: I183f670dc851de043e0a16d6060c069a9393ec99
Reviewed-by: Daniel Teske <daniel.teske@digia.com>