Add libssl-dev for Ubuntu

Rebuilding pyOpenSSL requires the SSL development libraries. This is
present in files/rpms/general, but was missed in files/apts/general.

Change-Id: Ie3c688e2d74485eb7f981363633bbddab5da7046
diff --git a/files/apts/general b/files/apts/general
index d65cab3..739fc47 100644
--- a/files/apts/general
+++ b/files/apts/general
@@ -25,3 +25,4 @@
 bc
 libyaml-dev
 libffi-dev
+libssl-dev # for pyOpenSSL