samples: Undo removal of include from canonicalize-app-names commit.

This include was accidentally removed in changeset
Ia79aea64de89531362e993e34230c2044a70aa93. My bad.

Change-Id: I1d716c7f9590b4e97909fb8bca1f2ed9bd0e4082

configs/samples/extensions.conf.sample

@@ -547,6 +547,7 @@ exten => s,n(fail),Hangup()
 
 
 [demo]
+include => stdexten
 ;
 ; We start with what to do when a call first comes in.
 ;