Update pg_buffercache's meson.build.

Commit 05d4cbf9b6 needed to do this,
but didn't.

Per Justin Pryzby.

Discussion: 20220927191710.GG6256@telsasoft.com

contrib/pg_buffercache/meson.build

@@ -11,6 +11,7 @@ install_data(
   'pg_buffercache--1.1--1.2.sql',
   'pg_buffercache--1.2--1.3.sql',
   'pg_buffercache--1.2.sql',
+  'pg_buffercache--1.3--1.4.sql',
   'pg_buffercache.control',
   kwargs: contrib_data_args,
 )