Turn mkunxarc.sh back into an ordinary sh script.

It became bash-dependent in r9229 because I used a bashism to remove
the 'r' from the front of $SVN_REV, but that's not needed any more.

[originally from svn r10281]
[r9229 == bd60f2fc]

mkunxarc.sh

-#!/bin/bash 
+#!/bin/sh
 
 # Build a Unix source distribution from the PuTTY CVS area.
 #