Change the return value of _Unwind_GetCFA in include/unwind.h.

I tested the original commit as part of a series that culminates in
removing this header and installing LLVM libunwind's unwind.h in its
place so missed updating this header as was done in b84693501af6.

Pointy hat to: jhb
Reported by: kevans
Fixes: 3a502289d316 Use uintptr_t for return type of _Unwind_GetCFA.