base-files: add /usr/share/libubox/jshn.sh to sysupgrade stage2

Discovered recent changes had broken sysupgrade for ar71xx mikrotik
rb-493g, traced the problem to missing /usr/share/libubox/jshn.sh after
switching to tmpfs.

Signed-off-by: Russell Senior <russell@personaltelco.net>

package/base-files/files/lib/upgrade/stage2

@@ -48,7 +48,7 @@ switch_to_ramfs() {
 		local file="$(which "$binary" 2>/dev/null)"
 		[ -n "$file" ] && install_bin "$file"
 	done
-	install_file /etc/resolv.conf /lib/*.sh /lib/functions/*.sh /lib/upgrade/*.sh /lib/upgrade/do_stage2 $RAMFS_COPY_DATA
+	install_file /etc/resolv.conf /lib/*.sh /lib/functions/*.sh /lib/upgrade/*.sh /lib/upgrade/do_stage2 /usr/share/libubox/jshn.sh $RAMFS_COPY_DATA
 
 	[ -L "/lib64" ] && ln -s /lib $RAM_ROOT/lib64