headers: use __linux__ instead of linux

headers: use __linux__ instead of linux

This file may be included by other programs, so it should be compliant
with the C standard.

* use __linux__ instead of linux - __linux__ is always defined, linux is
  not defined when gcc runs in standard-compliant mode (with -std=c89 or
  -std=c99) because the C standard doesn't allow polluting namespace
  with arbitrary defines.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>