Always print mutt_buffer_pool_free() size debug output.

So I can easily see the resulting size of the pool at the end of a
run.
4 jobs for master in 15 minutes and 29 seconds
Status Job ID Name Coverage
  Build
passed #115043020
gnutls

00:13:24

passed #115043019
openssl

00:14:11

 
  Deploy
passed #115043021
pages

00:01:18

passed pages:deploy

00:00:00