sanitize-commit: Fix warning from module import

'Attempt to call undefined import method with arguments ("path") via package
"File::Spec" (Perhaps you forgot to load the package?) at
.../qtrepotools/git-hooks/sanitize-commit line 15.'

It seems to come from File::Spec's API being intended as class methods,
so the whole module should be use'd.

Change-Id: Id279fae213e6ce11277067d8266e550740a63103
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>