Include: Test for __linux__

Explicitly test for __linux__ which is clearly the intent and
matches the behaviour described in the comments.

Signed-off-by: Nick Connolly <nick.connolly@mayadata.io>
Change-Id: I498476f4a214891b897609924e018c5b132afaf2
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/5143


Community-CI: Mellanox Build Bot
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>