Modify folly::coro's Malloc.cpp to support iOS <10.0

Summary:
Sized deallocation (introduced in C++14) is only available on iOS after 10.0.
This diff changes `folly_coro_async_free` to use `operator delete(void*)` when
`operator delete(void*, size_t)` is unavailable.

Reviewed By: yfeldblum

Differential Revision: D21492467

fbshipit-source-id: 2afef1d3d2aa6c70fec5f49e5d62a4a594e6da83

folly/experimental/coro/detail/Malloc.cpp

@@ -34,7 +34,13 @@ void* folly_coro_async_malloc(std::size_t size) {
 
 FOLLY_NOINLINE
 void folly_coro_async_free(void* ptr, std::size_t size) {
+#if __cpp_sized_deallocation >= 201309
   ::operator delete(ptr, size);
+#else
+  // sized delete is not available on iOS before 10.0
+  (void)size;
+  ::operator delete(ptr);
+#endif
 
   // Add this after the call to prevent the compiler from
   // turning the call to operator delete() into a tailcall.