asan_win_dynamic_runtime_thunk.cc: declare atexit

MSVC apparently makes atexit available even without including stdlib.h,
but clang-cl does not. This makes the file build also with clang-cl.

llvm-svn: 255160

compiler-rt/lib/asan/asan_win_dynamic_runtime_thunk.cc

@@ -59,6 +59,7 @@ int __asan_option_detect_stack_use_after_return =
 // using atexit() that calls a small subset of C terminators
 // where LLVM global_dtors is placed.  Fingers crossed, no other C terminators
 // are there.
+extern "C" int __cdecl atexit(void (__cdecl *f)(void));
 extern "C" void __cdecl _initterm(void *a, void *b);
 
 namespace {