zstd: upgrade to v1.4.0

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>
(cherry picked from commit 2c2797a71f7aca8f86a758237f1ecbc8966e1b51)

diff --git a/src/zstd b/src/zstd
index f4340f46b2387bc8de7d5320c0b83bb1499933ad..83b51e9f886be7c2a4d477b6e7bc6db831791d8d 160000 (submodule)
--- a/src/zstd
+++ b/src/zstd
@@ -1 +1 @@
-Subproject commit f4340f46b2387bc8de7d5320c0b83bb1499933ad
+Subproject commit 83b51e9f886be7c2a4d477b6e7bc6db831791d8d