dtoolutil: Use string compare instead of strcmp in filename.I

dtool/src/dtoolutil/filename.I

@@ -594,7 +594,7 @@ operator < (const std::string &other) const {
  */
 INLINE int Filename::
 compare_to(const Filename &other) const {
-  return strcmp(_filename.c_str(), other._filename.c_str());
+  return _filename.compare(other._filename);
 }