windows: fix mingw32 build, define JOB_OBJECT_XXX
windows: fix mingw32 build, define JOB_OBJECT_XXX

mingw32 doesn't have the JOB_OBJECT_XXX defines. Provide them ourselves
when they are not already defined.

This is possibly only an issue with older versions of mingw32 because
mingw-w64 2.0 _does_ export the defines.

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