Add python3 shebang

Python3 is required for this, so add the shebang so that people have a
chance of knowing that.

Tested By:
Launched tool with ./rootfs_size.py and observed that it launched
correctly.

Change-Id: I88f4d6a506809a48f6688a11128536b8eca842a0
Signed-off-by: Ed Tanous <ed.tanous@intel.com>
diff --git a/edtanous/rootfs_size.py b/edtanous/rootfs_size.py
index 7d6ef85..c7942c3 100755
--- a/edtanous/rootfs_size.py
+++ b/edtanous/rootfs_size.py
@@ -1,3 +1,5 @@
+#!/usr/bin/python3
+
 import subprocess
 import tempfile
 import os