Add cryptography to list if files to build before hand

building this takes 20 seconds or so

Change-Id: I95c71b1d0255c02038006bc743125ff2c49d9da9
diff --git a/files/venv-requirements.txt b/files/venv-requirements.txt
index 3c50061..8417b92 100644
--- a/files/venv-requirements.txt
+++ b/files/venv-requirements.txt
@@ -1,3 +1,4 @@
+cryptography
 lxml
 MySQL-python
 netifaces