fix zstd macro (#885)

Summary:
detect FOLLY_HAVE_LIBZSTD by true/false
Pull Request resolved: https://github.com/facebook/folly/pull/885

Reviewed By: terrelln

Differential Revision: D8807276

Pulled By: yfeldblum

fbshipit-source-id: 4372b925a487a31b26cc4c59d7f513639d864f00

folly/compression/Compression.cpp

@@ -1491,7 +1491,7 @@ bool LZMA2StreamCodec::doUncompressStream(
 }
 #endif // FOLLY_HAVE_LIBLZMA
 
-#ifdef FOLLY_HAVE_LIBZSTD
+#if FOLLY_HAVE_LIBZSTD
 
 static int zstdConvertLevel(int level) {
   switch (level) {