ptserver: Use correct structure type in malloc

prentries is a list of 'struct prlistentries' objects, not a list of
'struct prentry'. Use the correct type in our call to malloc.

Caught by clang analyzer

Change-Id: I6c36e4b875eafb8aff6506cf800d47b45a79825d
Reviewed-on: http://gerrit.openafs.org/9150
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>