'Build-up play' is a footballing term where a team doesn't just hoof it upfield like back in the good old days but instead spends time on the ball, working it forward from defence, through the midfield, into the attacking areas, usually probing for opprtunities to open up.
It doesn't make much sense to me here because Firmino can't really do this on his own. I'd guess it just means that Firmino was instrumental in whatever build-up led to Salah's goal.