BUG: Fix logic of LabelFiles.txt generation

This fixes a dumb logic error which causes generation of LabelFiles.txt
to try to open the file once for every target with labels.

Source/cmGlobalGenerator.cxx

@@ -2145,6 +2145,7 @@ void cmGlobalGenerator::WriteTargetLabels()
       if(!opened)
         {
         fout.Open(fname.c_str());
+        opened = true;
         }
       fout << file << "\n";
       }