qdoc: Clear the list of output files during generator init

QDoc keeps a list of files it generates (or copies) to the output
directory, for writing it to a .qhp file later on.

In single-exec mode, QDoc processes several qdocconf files
without deleting the generator in between. Therefore, we need to
clear the list of files whenever the generator is initialized,
to avoid duplicating the filenames across multiple .qhp files.

Change-Id: Ibc2a6b171466aa1db6cfe3da9a820d5ba2845004
Reviewed-by: Martin Smith <martin.smith@digia.com>

src/tools/qdoc/generator.cpp

@@ -1516,6 +1516,7 @@ void Generator::initialize(const Config &config)
     if (config.getBool(QString("HTML.nosubdirs")))
         resetUseOutputSubdirs();
 
+    outFileNames_.clear();
     outputFormats = config.getOutputFormats();
     redirectDocumentationToDevNull_ = config.getBool(CONFIG_REDIRECTDOCUMENTATIONTODEVNULL);
     if (!outputFormats.isEmpty()) {