portability: add missing headers
portability: add missing headers

libi3.h defins a macors with the default modes to create a directory.
Those modes are defined in sys/stat.h, so include the necessary header

i3/i3,stapelberg/i3,stapelberg/i3,i3/i3,Airblader/i3,Airblader/i3,stapelberg/i3,Airblader/i3,i3/i3,Airblader/i3,i3/i3,stapelberg/i3