Python: make the import of RedosUtil public

This mirrors `SuperlinearBacktracking.qll`
An alternative is to keep it private and import it again
in the query files.

python/ql/src/semmle/python/security/performance/ExponentialBackTracking.qll

@@ -62,7 +62,7 @@
  *     a suffix `x` (possible empty) that is most likely __not__ accepted.
  */
 
-private import ReDoSUtil
+import ReDoSUtil
 
 /**
  * Holds if state `s` might be inside a backtracking repetition.