Another take on fixing building issue on Macs

This patch is based on artosx's feedback on GitHub:

https://github.com/AltraMayor/f3/issues/6#issuecomment-67075953
Address unused-parameter warning on Macs

Function posix_fadvise(), only implemented on Macs, was issuing
warnings because the code doesn't use parameters 'offset' and 'len'.