[SL.Core] Silence warning.

src/SixLabors.Core/HashCode.cs

@@ -44,6 +44,7 @@ https://raw.githubusercontent.com/Cyan4973/xxHash/5c174cfa4e45a42f94082dc0d4539b
 
 #if SUPPORTS_HASHCODE
 using System.Runtime.CompilerServices;
+
 [assembly: TypeForwardedTo(typeof(System.HashCode))]
 #else
 using System.Buffers.Binary;