Install gcc on zuul hosts

With the addition of yappi, it's important to install gcc, because
otherwise yappi can't install.

Change-Id: Iaf19ab791f674bcbe5538ae8c7a18e25eef4ca99

modules/zuul/manifests/init.pp

@@ -50,6 +50,7 @@ class zuul (
   include pip
 
   $packages = [
+    'gcc',  # yappi requires this to build
     'python-webob',
     'python-lockfile',
     'python-paste',