gitk: chmod +x po2msg.sh

The Makefile only runs it using tclsh, but because the fallback po2msg
script has the usual tcl preamble starting with #!/bin/sh it can also
be run directly.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>