loop: Use signed type for fp_get_pollfds() retval

This will be a warning with slightly older versions of libfprint, but at
least it would catch errors.
2 jobs for master in 3 minutes and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #6040
build_dev

00:03:16

passed #6039
build_stable

00:03:01