buildroot

Revert "zstd: install to staging directory"

This reverts commit 95c15aaf15f4bd5b1ebcf87d204ddf5a345197d5. It was
mistakenly pushed, and causes problems because it installs the shared
library to staging, but not to target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>