opencl: Add a copy of cl.hpp 1.2 from khronos.org

opencl-headers of Debian stretch+ and Ubuntu wily+ no longer carry
cl.hpp. Mac OSX Xcode also does not have cl.hpp.

Use a local copy to avoid asking users to download cl.hpp which
requires root to install and may break API beyond control of
libfreenect2.

This updated local copy will also solve compiling errors
"_mm_mfence not declared" in #139 and #250.

Author: xlz