Do not build the Plugin Interface for unsupported platforms (ie. !linux).

Do not build the Plugin Interface for unsupported platforms (ie. !linux).

This will ensure the Plugin Interface is only run on the Linux platform
as this is the only validated platform for the Plugin Interface thus far.

Change-Id: I14507336d54a47cc32102c76cd1d4f1891cb3687
Signed-off-by: Joseph Morrow <joseph.l.morrow@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2785
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sachin Agrawal <sachin.agrawal@intel.com>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>