Fixed portabiliy issue in pre-commit hook

A change was previously applied to pre-commit via 1d4530d that
included a bashism that is not portable with other shells.

Change-Id: I0ab536005973627953e1a3b4fa41f51963a953ea
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/84413
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Reviewed-by: Luis P Fernandez <luis.fernandez@ibm.com>
Tested-by: Jenkins OP Build CI <op-jenkins+hostboot@us.ibm.com>
Tested-by: FSP CI Jenkins <fsp-CI-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins OP HW <op-hw-jenkins+hostboot@us.ibm.com>
Reviewed-by: Daniel M Crowell <dcrowell@us.ibm.com>

src/build/tools/pre-commit

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # IBM_PROLOG_BEGIN_TAG
 # This is an automatically generated prolog.
 #