xenomai/pepare-kernel.sh: switch to using --outpatch=

generating the patch with --outpatch=<path>, and applying it thereafter
resolves a dangling links issue under /usr/src/linux-headers*
Merge pull request #4 from mhaberler/3.8.13-xenomai-outpatch

xenomai/pepare-kernel.sh: switch to using --outpatch=