Promote libffi-dev as a general dependency

libffi-dev/libffi-devel can be requested for rebuilding the
python clients which is an un-skipable part of the devstack install.

Promoting the libffi-devel to a general dependency.

Change-Id: I1dac7fd536b94de1e49aaf335ac29ffae1bd39d1
diff --git a/files/rpms/general b/files/rpms/general
index a0074dd..74997a8 100644
--- a/files/rpms/general
+++ b/files/rpms/general
@@ -7,6 +7,7 @@
 openssh-server
 openssl
 openssl-devel # to rebuild pyOpenSSL if needed
+libffi-devel
 libxml2-devel
 libxslt-devel
 psmisc