build_tools/dkms-helper: Use bash to process dkms-helper script.

Looks like there are some bashims in the script. Make this explicit in the
script until they can be taken out.

This fixes failures to run this script on Ubuntu when dash is the default shell.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>
(cherry picked from commit 41b5353338)

build_tools/dkms-helper

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 set -e