make: build common/secret only on linux

libkeyutils is only available on linux. before the automake refactoring
secret.c was compiled into rbd and ceph_mount targets which are linux
only targets. secret.c was moved to libcommon during the refactoring,
but the conditional compilation was lost.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #624 from ceph/automake-fix-common

make: build common/secret only on linux

Reviewed-by: Sage Weil <sage@inktank.com>