win32: assume PROCESS_MITIGATION_IMAGE_LOAD_POLICY is defined

It was added in mingw-w64 in [1] which is in v6, our minimum version. We don't need to have it defined in UWP builds as we don't use winvlc.c.

[1] https://github.com/mingw-w64/mingw-w64/commit/36d7b92bbcec1e72d3ce24013b01f7acc34be3b0