Include unistd.h

Description
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Includes `unistd` in the `SaliencyDetector.cpp` file.

Explanation
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In order for this to compile under Debian using g++ version `g++ (Debian 4.7.2-5) 4.7.2` I had to add `unistd.h` to the `src/SaliencyDetector.cpp` otherwise the `access` function and its constants (`F_OK`, here) were not defined.

I'm not a C or a C++ guy generally so this may not be appropriate for a merge but it worked for me.