security/strongswan: Add GCM option to OPTIONS_DEFAULT

Avoid the message:

"plugin 'gcm': failed to load - gcm_plugin_create not found and no
plugin file available"

According to strongSwan's 5.9.8 release notes[1]:

The gcm plugin has been enabled by default, so that the TLS 1.3 unit
tests (now indirectly enabled if the pki tool is built due to the
implementation of EST) can be completed successfully with just the
default plugins.

Let's also enable it by default.

[1]: https://github.com/strongswan/strongswan/releases/tag/5.9.8

PR: 267352