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 <ff46fec9739d31cce915aabcc3a7a2c00eee4d6c@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/19327
Reviewed-by: Alex Kelley <b715314911e4c2b690fc64ea4cf8f2f989d18c99@microsoft.com>
Reviewed-by: Dan Mihai <ba8e1833495515b49ded9e20cae80a8129155aa9@microsoft.com>
Tested-by: Dan Mihai <ba8e1833495515b49ded9e20cae80a8129155aa9@microsoft.com>

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