Fix wide-char use in Posix collator

Although the C++ language builds in the 'wchar_t' datatype, the
library functions such as wcscmp() which manipulate them are not
automatically available. For these, inclusion of the <cwhar> header
is still required.

This changeset fixes build breakage observed from failure to include
the requisite system header for accessing wcscmp() and other related
functions on non-GNU standard C++ library implementations.

Change-Id: I5b2f9148ea011004e5dd00cf41698339db172de8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>