ManagedBuffer.capacity is unavailable on OpenBSD.

`.capacity` is backed by malloc introspection (`malloc_size`), which is
not available on all platforms. The workaround is straightforward here,
thankfully: instead, use the minimum capacity already specified.

Author: 3405691582

Sources/System/Internals/RawBuffer.swift

@@ -80,7 +80,13 @@ extension _RawBuffer {
     internal static func create(minimumCapacity: Int) -> Storage {
       Storage.create(
         minimumCapacity: minimumCapacity,
-        makingHeaderWith: { $0.capacity }
+        makingHeaderWith: {
+#if os(OpenBSD)
+          minimumCapacity
+#else
+          $0.capacity
+#endif
+        }
       ) as! Storage
     }
   }