Add tokens for cl_img_use_gralloc_ptr, cl_img_cached_allocations and  cl_img_yuv_image extensions (#13)
Add tokens for cl_img_use_gralloc_ptr, cl_img_cached_allocations and  cl_img_yuv_image extensions (#13)

* Add tokens for cl_img_use_gralloc_ptr, cl_img_cached_allocations and
cl_img_yuv_image extensions

* Reinstate accidentally removed extern C

KhronosGroup/OpenCL-Headers,KhronosGroup/OpenCL-Headers