dnsdist: Only declare/set freshConn if MSG_FASTOPEN is defined

[rgacogne]

Short description

Otherwise the compiler rightly warns about the variable being set but not used when MSG_FASTOPEN is not available:

warning: variable 'freshConn' set but not used

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)