Issues with cl_h.rs in the cl-sys crate

[Ken Barker]

There are a few minor issues in cl_h.rs, they are:

• line 735 clGetDeviceAndHostTimer: device_timestamp and host_timestamp should be mutable pointers.
• line 744 clGetHostTimer: host_timestamp should be a mutable pointer.
• line 972 clCreateProgramWithBuiltInKernels: kernel_names should be a const pointer not a mutable pointer

Also, if the 3 structs: cl_image_format, cl_image_desc, and cl_buffer_region were declared #[derive(Debug)] it would make them easier to use.