[contrib][linux] Fix build with CONFIG_WERROR

Linux 5.15 introduces a new Kconfig option, CONFIG_WERROR, which
forces -Werror for the entire kernel.
Current in-kernel ZSTD implementation uses functions deprecated
in 1.5.0, and thus fails on -Wdeprecated-declarations.
Turn this particular error into warning to be able to build the
kernel with CONFIG_WERROR. I'm not disabling them completely to
make sure they'll be visible and [hopefully] fixed sooner or later.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>