swaybg: make argv const
swaybg: make argv const

SirCmpwn/sway,sleep-walker/sway,1ace/sway,mikkeloscar/sway,ascent12/sway,colemickens/sway,christophgysin/sway,christophgysin/sway,crondog/sway,taiyu-len/sway,4e554c4c/sway,crondog/sway,mikkeloscar/sway,1ace/sway,taiyu-len/sway,taiyu-len/sway,ascent12/sway,johalun/sway,colemickens/sway,colemickens/sway,1ace/sway,ptMuta/sway,ascent12/sway,4e554c4c/sway,sleep-walker/sway