ads2gas.pl: Don't require perl to be in a fixed location

This reduces the differences to ads2gas_apple.pl.

Change-Id: Ie30e369e3685b35485efe2296f7cdf91dafee1e7
diff --git a/build/make/ads2gas.pl b/build/make/ads2gas.pl
index 9c41901..7272424 100755
--- a/build/make/ads2gas.pl
+++ b/build/make/ads2gas.pl
@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 ##
 ##  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
 ##