Bash is not guaranteed to be in /usr/pkg/bin/bash.

On my FreeBSD 11 VM, it's the standard FreeBSD package, which installs
it in /usr/local/bin/bash.

Change-Id: Ic6427a88efd963de8909a2d333c2c3bf8586e559
Reviewed-on: https://code.wireshark.org/review/29211
Reviewed-by: Guy Harris <guy@alum.mit.edu>

tools/bsd-setup.sh

@@ -1,4 +1,4 @@
-#!/usr/pkg/bin/bash
+#!/usr/bin/env bash
 # Setup development environment on BSD-like platforms.
 #
 # Tested on: FreeBSD, OpenBSD, NetBSD.