Add libyaml-devel package dependancy

Without libyaml-devel package installed (on Fedora-20),
running ./stack throws:

  [. . .]
  build/temp.linux-x86_64-2.7/check_libyaml.c:2:18: fatal error: yaml.h:
  No such file or directory #include <yaml.h> ^ compilation terminated.

  libyaml is not found or a compiler error: forcing --without-libyaml
  [. . .]

Adding this dependency to files/rpms/general resolves it. Also added to
files/apts/general.

Change-Id: If1aa23bf3fc76ea2b321c3725dd07050ddd4b825
Merge "Add libyaml-devel package dependancy"
Merge branch 'master' of github.com:openstack-dev/devstack