fix build on mingw

It's beyond me why mingw isn't recognizing the UNICODE macro at the top of the file.  I even went as far as looking at the _mingw.h (where __AW is defined, which should be selecting either -A or -W suffixed functions, based on UNICODE).  Inspecting the gcc -E output, it looks like all the requisite macros are defined / undefined properly to allow us to take the -W branch of the `#if`.  I'm stumped

The workaround is to add an explicit -W suffix to the method call in question.