Gerbview, read zip archive: do not generate a error message for directory names.

A directory is not a file, and is now just skipped.

gerbview/files.cpp

@@ -463,6 +463,9 @@ bool GERBVIEW_FRAME::unarchiveFiles( const wxString& aFullFileName, REPORTER* aR
 
     while( ( entry = zipArchive.GetNextEntry() ) )
     {
+        if( entry->IsDir() )
+            continue;
+
         wxString   fname = entry->GetName();
         wxFileName uzfn = fname;
         wxString   curr_ext = uzfn.GetExt().Lower();