Fix charmcraft pack for keystone charm

Add missing cryptography dependencies.

Change-Id: Id933b7b83ad81014bdbd2832379d202e8bc07e3f
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>
diff --git a/installers/charm/keystone/charmcraft.yaml b/installers/charm/keystone/charmcraft.yaml
index 0a285a9..04b659a 100644
--- a/installers/charm/keystone/charmcraft.yaml
+++ b/installers/charm/keystone/charmcraft.yaml
@@ -34,4 +34,8 @@
           - arm64
 parts:
   charm:
-    build-packages: [git]
+    build-packages:
+      - git
+      - libffi-dev
+    charm-binary-python-packages:
+      - cryptography