fixed warning in folly::align_ceil

Summary:
Fix the following warning on windows with msvc:

```
folly/lang/Align.h(222,35): warning C4146: unary minus operator applied to unsigned type, result still unsigned
```

Reviewed By: Orvid

Differential Revision: D61764439

fbshipit-source-id: f5eb9a51c6c81c4965cb4f7c11f0971aa99df950

folly/lang/Align.h

@@ -219,7 +219,8 @@ struct align_ceil_fn {
   constexpr std::uintptr_t operator()(
       std::uintptr_t x, std::size_t alignment) const {
     detail::validateAlignment(alignment);
-    return (x + alignment - 1) & (-alignment);
+    auto alignmentAsInt = static_cast<std::intptr_t>(alignment);
+    return (x + alignmentAsInt - 1) & (-alignmentAsInt);
   }
 
   template <typename T>