[pkg] add-apt-repository not installed by default

Makefile

@@ -34,6 +34,7 @@ all: icon locales build
 
 install_go:
 	# the version of go in bionic is too old. let's get something newer from a ppa.
+	@sudo apt install software-properties-common
 	@sudo add-apt-repository ppa:longsleep/golang-backports
 	@sudo apt-get update
 	@sudo apt-get install golang-go