[libunwind] Fix a few libunwind includes

[rprichard]

Summary

In UnwindCursor.hpp, include config.h before checking
_LIBUNWIND_SUPPORT_SEH_UNWIND.

Include libunwind_ext.h for UNW_STEP_SUCCESS.

[rprichard]

On second thought, maybe I don't actually need this change, and the includes still seem like a mess in libunwind. Maybe it makes more sense to do a larger revision eventually.

[compnerd]

This seems relatively harmless, not sure if its valuable though.

[MaskRay]

... so that it no longer shows on my review queue. I think this is still worth doing, especially the weird include places in UnwindCursor.hpp might confuse readers.