lib: Fix privs syscaps (pset_t) allocation

lib: Fix privs syscaps (pset_t) allocation

Don't over-allocate syscaps in zcaps2sys(): This is just a single struct
(pset_t) with a count and a pointer to an array of capabilities, not an
array. So only allocate a single pset_t, not num copies of it.

The allocation size of syscaps->caps then needs to be based on the number of
Linux capabilities (count), but that is already handled properly a few lines
below.

Note that this fix is mostly cosmetic and for correctness. There was no
potential for memory corruption, because num is guaranteed to be nonzero. So
at least the one required pset_t was always allocated (but potentially much
more).

Signed-off-by: Martin Buck <mb-tmp-tvguho.pbz@gromit.dyndns.org>