[CHEF-1312] correctly include a mixin.

While here, also require the file the mixin comes from, for correctness sake.

libraries/default.rb

@@ -18,6 +18,8 @@
 # limitations under the License.
 #
 
+require "chef/mixin/from_file"
+
 class Chef
   class Resource
     # Globally update the blocklists to prevent infinite recursion in #to_json and similar
@@ -128,7 +130,7 @@ class Chef
     module ApplicationBase
 
       def self.included(klass)
-        klass.extend Chef::Mixin::FromFile
+        klass.send(:include, Chef::Mixin::FromFile)
       end
 
       def deploy_provider