Correct statvfs type casts for OpenBSD.

On OpenBSD, fsblkcnt_t -- the type of f_blocks -- is a UInt64; therefore,
so must `blockSize` be.

Ultimately, both sides of the `totalSizeBytes` multiplication should
probably be type cast for all platforms, but that's a more significant
functional change for another time.

Author: 3405691582

Sources/FoundationEssentials/FileManager/FileManager+Files.swift

@@ -733,6 +733,9 @@ extension _FileManagerImpl {
             #if canImport(Darwin)
             let fsNumber = result.f_fsid.val.0
             let blockSize = UInt64(result.f_bsize)
+            #elseif os(OpenBSD)
+            let fsNumber = result.f_fsid
+            let blockSize = UInt64(result.f_bsize)
             #else
             let fsNumber = result.f_fsid
             let blockSize = UInt(result.f_frsize)