Rename static function to avoid conflicting names

Rename static function to avoid conflicting names

Commit a4fd3aa719e moved setup_cancel_handler out of psql and
exporeted it as a global function.  While pg_dump isn't using
the header it's exported in,  having a conflicting name still
risks causing confusion when grepping the code for callsites,
so rename the static function in pg_dump to avoid this.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Discussion: https://postgr.es/m/20240126094245.cf6718cc659273765f3ab69a@sraoss.co.jp