tools/stack_decode.py: Use 'python2' instead of 'python'

Since the script does not work with python3.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>

tools/stack_decode.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 # Call addr2line as needed to resolve addresses in a stack trace. The addresses
 # will be replaced if they can be resolved into file and line numbers. The