pg_basebackup: Fix comparison handling of tablespace mappings on Windows

A candidate path needs to be canonicalized before being checked against
the mappings, because the mappings are also canonicalized.  This is
especially relevant on Windows

Reported-by: nb <nbedxp@gmail.com>
Author: Michael Paquier <michael.paquier@gmail.com>
Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>