[Win32] Change libcoap bitfield structure to use ushort
[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 <51925a00f4213b9c3a04d90d96c6318365ad0565@intel.com>
Signed-off-by: Daniel Ferguson <5f41060945303b8d0295214bd188aa419b8121b5@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5511
Tested-by: jenkins-iotivity <09cb29e8a2b473a2c978382eec13ee06fa017bda@opendaylight.org>
Reviewed-by: David Antler <847c7e7639e695cd03ee85957de4a1178f752d1b@intel.com>

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