IPCA: Declare class in namespace

IPCA: Declare class in namespace

Issue was reported using clang-3.5:

  resource/IPCA/src/inc/callback.h:39:7:
  error: forward declaration of class cannot have a nested name specifier
  class OC::OCResource;

Change-Id: I1dd8c37a116b7f281aecb3f1c24700af69bf2b88
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/19327
Reviewed-by: Alex Kelley <alexke@microsoft.com>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Tested-by: Dan Mihai <Daniel.Mihai@microsoft.com>