get.sh: add missing check in fetch_project()

we check the main url, but not backup urls.

this patch fixes that oversight.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/get.sh

@@ -22,7 +22,7 @@ fetch_project()
 	eval "`setvars "" xtree`"
 	eval "`setcfg "config/git/$project/pkg.cfg"`"
 
-	chkvars url
+	chkvars url bkup_url
 
 	[ -n "$xtree" ] && x_ ./mk -f coreboot "$xtree"
 	[ -z "$depend" ] || for d in $depend ; do