Fix compilation of openni2 on OS X 10.9.

See issue https://github.com/ros/homebrew-hydro/issues/10

Upstream openni apparently addresses this, so it should be ok to
remove the patch after the next openni release.
Merge pull request #11 from NikolausDemmel/fix-openni-10.9

Fix compilation of openni2 on OS X 10.9.