Make sure rabbit_prelaunch_conf:decrypt_config/2 is available to tests

This fixes an `undef` failure in unit_config_value_encryption_SUITE.

On more recent branches starting with `v4.0.x` this
change was applied as part of a larger commit,
e.g. cfa3de4.

Author: michaelklishin

deps/rabbitmq_prelaunch/src/rabbit_prelaunch_conf.erl

@@ -12,9 +12,8 @@
          generate_config_from_cuttlefish_files/3,
          decrypt_config/1]).
 
--ifdef(TEST).
+%% Only used in tests.
 -export([decrypt_config/2]).
--endif.
 
 %% These can be removed when we only support OTP-26+.
 -ignore_xref([{user_drv, whereis_group, 0},