ns-plug: mwan3 hooks needs bash (#993)

The script uses a bash-only syntax. Using bash gives the following error:
/usr/libexec/ns-plug/mwan-hooks: line 17: failed_count++: not found

#1014

Author: filippocarletti

packages/ns-plug/files/mwan-hooks

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 #
 # Copyright (C) 2024 Nethesis S.r.l.