test/symbols-leak-test: use more portable shebang

Some BSDs don't want to give bash the honor of /bin and put it
elsewhere. So look it up in PATH instead.

Signed-off-by: Ran Benita <ran234@gmail.com>

test/symbols-leak-test.bash

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 # Check that all exported symbols are specified in the symbol