Add missing void in named_config_getdefault() definition

The named_config_getdefault() was missing void in the function
definition.  This broke clang-15 that didn't match the declaration that
had the void in the argument with the definition that hadn't.
66 jobs for !6259 with ondrej-add-missing-void-in-named_config_default in 3 minutes and 51 seconds (queued for 3 seconds)
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