Add includedir for Gentoo

This breaks otherwise for setups using STRICT_VARIABLES=yes

Author: baurmatt

Diff for: manifests/params.pp

@@ -256,6 +256,7 @@
 
     'Gentoo': {
       $client_package_name = 'virtual/mysql'
+      $includedir          = undef
       $server_package_name = 'virtual/mysql'
       $basedir             = '/usr'
       $config_file         = '/etc/mysql/my.cnf'