Please unblock package libzstd

[ Reason ]
Version 1.5.4+dfsg2-5 in unstable runs a couple more tests
during the build (which is not that important), and also
includes fixes for two bugs, one of them leading to data
corruption in very rare cases; see #1034012 for details
(that bug was filed by somebody else today, it is not closed
by version 1.5.4+dfsg2-4 that included the patch).

[ Impact ]
Programs using libzstd (including dpkg) run a small chance of
data corruption.

[ Tests ]
The patch includes an addition to the build-time zstd
test suite:

https://salsa.debian.org/pkg-rpm-team/libzstd/-/blob/master/debian/patches/0020-upstream-fix-block-splitter.patch#L48

[ Risks ]
IMHO the patch is straightforward for people with an understanding
of the C programming language, so the risks are minimal.

[ Checklist ]
  [x] all changes are documented in the d/changelog
  [x] I reviewed all changes and I approve them
  [x] attach debdiff against the package in testing

[ Other info ]

unblock libzstd/1.5.4+dfsg2-5

Unblocked.