SjLj: Fix building after SVN r314632

The code moved from Unwind_AppleExtras.cpp to Unwind-sjlj.c needed
a few minor modifications to build as C instead of C++.

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@314635 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed