linux: fix a undefined warning about hwloc_linu...

linux: fix a undefined warning about hwloc_linux_parse_cpuset_file()

Move it in the same #ifdef as its only caller.

This commit was SVN r5786.

(cherry picked from commit 188bded57107e9b8ed9dd3373b122c00ee2e28ec)
linux: /proc/device-tree will move to /sys/firmware/devicetree/base in 3.15

/proc/device-tree will still remain as a symlink fortunately.

(cherry picked from commit df844ff2f593f241accb4e80f3472330e0c74501)