Include unistd.h in svgview.cpp (required for getcwd() function)

This fixes compilation on Arch Linux, which otherwise fails.

Signed-off-by: Martin Schmölzer <martin.schmoelzer@student.tuwien.ac.at>
Merge pull request #17 from mschmoelzer/master

Include unistd.h in svgview.cpp (required for getcwd() function)