Added a missing -lpthread flag to one Makefile

  The lack of the -lpthread flag on Source/Tools/NiViewer/Makefile prevented the
  whole project from compiling correctly on Ubuntu 14.04 64 bit. Just added the
  missing flag to this Makefile, according to the same solution for issue 47 of
  the old OpenNI2 Github repository, that boils down to a similar problem and
  can be found here, on Github
  https://github.com/OpenNI/OpenNI2/issues/47