Don't include stdalign.h in refcount.c

Instead, rely on internal.h to either include the header or polyfill it.
On Windows, we don't (yet) require C11, so we can't rely on the header
being directly includable. Though given it took a couple months to
notice this, it's clear the non-C11 path is pretty much untested, so we
need to get rid of it.

Bug: 624
Change-Id: I86a6961c93161c3adfacd374affb8bfb2be0a542
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/61445
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>

crypto/refcount.c

@@ -15,7 +15,6 @@
 #include "internal.h"
 
 #include <assert.h>
-#include <stdalign.h>
 #include <stdlib.h>