Don't assume std namespace

The PFW core headers have been reworked to remove all occurences of "using
namespace std", which is regarded as bad practice leading to namespace
poisoning.

The filesystem plugin needs to be updated accordingly.

Change-Id: I5077b1f60b3a8b773031ba8a0c9ae924be8ffdd9
Signed-off-by: David Wagner <david.wagner@intel.com>