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
diff --git a/src/Unwind-sjlj.c b/src/Unwind-sjlj.c
index 4206c46..90cac3f 100644
--- a/src/Unwind-sjlj.c
+++ b/src/Unwind-sjlj.c
@@ -67,7 +67,7 @@
 #if defined(__APPLE__)
 #include <System/pthread_machdep.h>
 #else
-static _LIBUNWIND_THREAD_LOCAL _Unwind_FunctionContext *stack = nullptr;
+static _LIBUNWIND_THREAD_LOCAL struct _Unwind_FunctionContext *stack = NULL;
 #endif
 
 static struct _Unwind_FunctionContext *__Unwind_SjLj_GetTopOfFunctionStack() {