[Win32] Change libcoap bitfield structure to use ushort

* unsigned int bitfields compile differently between GCC 4.8.4 on Linux
  and GCC 5.2.0 in msys2. Visual Studio's C Compiler Version 19 builds
  the same way as the msys2 GCC.
* Specifiying unsigned short for all platforms does not change the
  structure on Linux, but it does fix the msys2 and VS structures.

Change-Id: I5ef10c0485413859a3f93c67ecb3c148a828b3de
Signed-off-by: Henry Beberman <henry.beberman@intel.com>
Signed-off-by: Daniel Ferguson <daniel.j.ferguson@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5511
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: David Antler <david.a.antler@intel.com>