Added constructors to trivial struct types used in C++

Added constructors to trivial struct types used in C++

The C++ stack uses a OCHeaderOption and OCCallbackData
directly and in situations where construction is necessary.

This patch adds a constructor to both (a trival ctor, permitted
by C++11) POD types.  This cleans up the code and prevents
avoidable errors with either.

Change-Id: I744daf3bb091b646e06d23a069bbe7ee048a8e51
Signed-off-by: Erich Keane <erich.keane@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/1100
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>