Define OC_CALL (call convention for public APIs).

Define OC_CALL (call convention for public APIs).

Set to __stdcall for WIN32.
Add to IPCA public APIs.
Other public APIs will be updated in separate commit.

Change-Id: I3201b7088eeb2227d2681847d20172825fbf4cae
Signed-off-by: Soemin Tjong <stjong@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18169
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Way Vadhanasin <wayvad@microsoft.com>
Reviewed-by: Mike Fenelon <mike.fenelon@microsoft.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>