zstd: upgrade to v1.4.5

Since the v1.4.0 release there have been a few improvements to Zstandard
including improved compression ratios, faster compression, and faster
decompression.

Signed-off-by: Bryan Stillwell <bstillwell@godaddy.com>

diff --git a/src/zstd b/src/zstd
index 83b51e9f886be7c2a4d477b6e7bc6db831791d8d..b706286adbba780006a47ef92df0ad7a785666b6 160000 (submodule)
--- a/src/zstd
+++ b/src/zstd
@@ -1 +1 @@
-Subproject commit 83b51e9f886be7c2a4d477b6e7bc6db831791d8d
+Subproject commit b706286adbba780006a47ef92df0ad7a785666b6