Write a newline after adding dynamic_shared_memory_type to PG conf (#9843)

Without adding a newline, we can end up with a conf line that looks like
the following:

dynamic_shared_memory_type = mmap# Managed by compute_ctl: begin

This leads to Postgres logging:

LOG: configuration file
"/var/db/postgres/compute/pgdata/postgresql.conf" contains errors;
unaffected changes were applied

Signed-off-by: Tristan Partin <tristan@neon.tech>

compute_tools/src/config.rs

@@ -116,7 +116,7 @@ pub fn write_postgres_conf(
                 vartype: "enum".to_owned(),
             };
 
-            write!(file, "{}", opt.to_pg_setting())?;
+            writeln!(file, "{}", opt.to_pg_setting())?;
         }
     }