upgrade libzstd submodule (1.4.5 -> 1.5.7)

This fixes a couple CVEs:
CVE-2021-24031
CVE-2021-24032

There doesn't appear to be changes to the API other than deprecating a
couple experimental functions

diff --git a/src/zstd b/src/zstd
index b706286adbba780006a47ef92df0ad7a785666b6..f8745da6ff1ad1e7bab384bd1f9d742439278e99 160000 (submodule)
--- a/src/zstd
+++ b/src/zstd
@@ -1 +1 @@
-Subproject commit b706286adbba780006a47ef92df0ad7a785666b6
+Subproject commit f8745da6ff1ad1e7bab384bd1f9d742439278e99