Include unistd.h in passes/hierarchy/hierarchy.cc (required for access(3))

This fixes compilation errors on Arch Linux.

Signed-off-by: Martin Schmölzer <martin.schmoelzer@student.tuwien.ac.at>
Merge pull request #20 from mschmoelzer/master

Include unistd.h in passes/hierarchy/hierarchy.cc (required for access(3))