Missing std prefix in skeleton plugin

The skeleton plugin is not compiled when compiling
the root pfw project. As a result, when `using namespaces`
was removed from the pfw headers the skeleton plugin
was left unchecked and unfortunately broken.

Fix the missing `std::` by adding them where needed or
inserting a `using std::string`.

Signed-off-by: Kevin Rocard <kevin.rocard@intel.com>