Fix for OpenCL headers on case sensitive filesystem.

On a Mac with a case sensitive file system (most people won't have this as it's not the default) then the OpenCL headers fail to be included by:

```#include <OpenCL/OpenCL.h>```

However, this works:

```#include <OpenCL/opencl.h>```

Please note this fixed the issue for me and I am now able to compile OpenCV on my mac. I don't have any other Macs to test on, and obviously someone will need to check that this doesn't break case insensitive filesystems.