QtAutogen: Don't take a reference to temporary.

While a const reference to a temporary is standard conformant,
it doesn't seem to be necessary or advantageous here.

Source/cmQtAutoGenerators.cxx

@@ -1500,7 +1500,7 @@ void cmQtAutoGenerators::Init()
 
     if (this->IncludeProjectDirsBefore)
       {
-      const std::string &binDir = "-I" + this->ProjectBinaryDir;
+      const std::string binDir = "-I" + this->ProjectBinaryDir;
 
       const std::string srcDir = "-I" + this->ProjectSourceDir;