Force subsurface.sh on Mac to use bash

This was in a patch set from Henrik but got dropped at first while we
explored a different solution. So now it comes back as maybe the most
trivial commit, ever :-)

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

packaging/macosx/subsurface.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 if test "x$GTK_DEBUG_LAUNCHER" != x; then
     set -x