Include necessary header files to get compilation on OSX

Hi,

   There were some missing 'include' statements to get the library to compile on OSX (tested on 10.9, these are really basic stuff, so I wouldn't expect them to have changed in a while). Source: https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man2/getpid.2.html

Thanks,

Adrien