Redhill: Add config-footer.font.line-height default variable.

redhill/sass/_config-child-theme-deep.scss

@@ -355,5 +355,6 @@ $config-footer: (
 	"font": (
 		"family": map-deep-get($config-global, "font", "family", "secondary"),
 		"size": map-deep-get($config-global, "font", "size", "sm"),
+		"line-height": map-deep-get($config-global, "font", "line-height", "sm"),
 	),
 );