Add libffi dev packages needed for ansible install

Ansible depends on paramiko which depends on cryptography whcih requires
libffi dev headers when installing via pip. Go ahead and add this
dependency via bindep.

Change-Id: I0b3ab07245f85c44ad4ed436da06d62a21a28f89

bindep.txt

@@ -0,0 +1,4 @@
+libffi-dev [platform:dpkg]
+libffi-devel [platform:rpm]
+libssl-dev [platform:dpkg]
+openssl-devel [platform:rpm]