[contrib][linux] Fix -Wundef inside Linux kernel tree

[contrib][linux] Fix -Wundef inside Linux kernel tree

Commit d7ef97a013b5
("[build] Fix oss-fuzz build with the dataflow sanitizer") broke
build inside Linux-kernel after 'import', as it no longer can
conditionally remove ZSTD_MEMORY_SANITIZER definition from
the #if DEF_A || DEF_B block. This emits -Wundef warning which
can be treated as error.
Split this preprocessor condition into two separate conditions
to fix this.

Fixes: d7ef97a013b5 ("[build] Fix oss-fuzz build with the dataflow sanitizer")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>